- Email: [email protected]
Joseph Gall—pioneering nuclear biology
PIONEERS
Joseph Gall – pioneering nuclear biology Mary Lou Pardue By the mid-1960s, spectacularly successful studies had revealed much of the molecular biology of bacteria and viruses – how their genes are transcribed, translated and replicated. The next question was to what extent this information could be applied to multicellular organisms. What would be different about eukaryotic cells? Very few people had started to attack those questions when I began looking for a place to do graduate work in eukaryotic molecular biology. The name that kept coming up in my search was Joseph Gall (Boxes 1 and 2). Joe was pioneering eukaryotic molecular biology in those days, and he still is today. A look at Joe’s career shows that he has continually moved at the forefront of the field, asking important questions and finding unexpected answers. He has also been innovative in developing BOX 1 – JOE GALL Joseph Gall received a PhD in Zoology from Yale in 1952. He joined the faculty of the University of Minnesota, rising to the rank of Professor. In 1964, he became the Professor of Biology (later the Ross Granville Harrison professor) at Yale, and in 1983 he moved to the Carnegie Institution in Baltimore, where he is now American Cancer Society Professor of Developmental Genetics. His many honours include membership in the National Academy of Sciences, Accademia Nazionale dei Lincei, Rome, American Academy of Arts and Sciences, and the American Philosophical Society. He has been President of both the American Society for Cell Biology and the Society for Developmental Biology.
Mary Lou Pardue is in the Dept of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. E-mail: mlpardue @MIT.EDU
208
and adapting new techniques to address problems in cell biology. In each case, the new technique has been immediately used to solve an interesting question. From the beginning, one of the hallmarks of Joe’s work has been his ability to choose experimental materials that are uniquely suited for study of the problem at hand. A childhood passion for insect collecting gave him a broad knowledge of natural history. This basis allowed him to identify organisms or cell types with useful ‘oddities’ and then to exploit these to understand more-general problems. Joe has worked on a wide variety of organisms, including amphibia, fruit flies, protozoa and ferns. His PhD thesis research is a good example of astute choice of material. Although he studied with Donald Poulson, a Drosophila geneticist, Joe chose to work on the lampbrush chromosomes of amphibian oocytes1. These are the largest chromosomes known, and the nuclei are big enough to be isolated manually. In addition, lampbrush chromosomes come from a cell that is in the midst of laying down the groundwork of much of the embryo and therefore allow us to observe some very important genetic activities (see Fig. 1 on page 207). Today, when we can detect a single gene with a nucleic acid probe and identify individual proteins with monoclonal antibodies, it is hard to realize how important chromosome size was for the early studies. The first question was how DNA, RNA and protein, whose functions were just being elucidated in studies on prokaryotes, were related to the structures that cytologists had described in nuclei of eukaryotic cells. The tools available were blunt – stains with some sort of specificity, in vivo incorporation of radioactive precursors or selective digestion by enzymes. All these tools required significant amounts of material to permit detection. DNA, for example, could be detected by the Feulgen staining reaction, yet the reaction produced a puzzling result when used on lampbrush chromosomes. As expected from studies on other, less remarkable, chromosomes, the long axes of the chromosomes showed positive staining. By contrast, the loops that project from the axes and give the lampbrush chromosomes their name, did not stain with Feulgen1. Other chromosomes did not show detectable loops (at least in the stages of the cell cycle when they are most amenable to cytological analysis), so there were no other studies to compare with this result. We now know that the non-staining of the lampbrush loops is due to the decondensed nature of the DNA in these regions; more-powerful new stains, such as DAPI, can detect DNA in the loops. At the time, however, the result only emphasized important existing questions about the nature of the projecting lampbrush loops and about whether chromosomes are single intact molecules of DNA. Joe’s experiments answered both of these questions. His studies of uridine incorporation2 showed that RNA synthesis occurs on the lampbrush loops, thus demonstrating for the first time a site of cellular RNA synthesis. An elegant analysis of the kinetics of DNase digestion of lampbrush chromosomes showed that the chromosome is a single DNA molecule, with lesscondensed regions forming the lampbrush loops, where RNA synthesis occurs3. Joe also took advantage
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0962-8924/98/$19.00 PII: S0962-8924(98)01247-1
trends in CELL BIOLOGY (Vol. 8) May 1998
pioneers BOX 2 – JOE GALL AND THE HISTORY OF CELL BIOLOGY
of the ease of isolating oocyte nuclei to obtain a look at the nuclear envelope, and his electron-microscopic studies established the existence of the octagonally symmetrical nuclear pore complex4. Amphibian oocytes have no obvious advantages for studying centriole replication. Therefore, to study this subject, Joe turned to spermiogenesis in a snail, Viviparous5, and sperm formation in a fern, Marsilea6. In both cases, the centriole replicates multiple times to give rise to the basal bodies for the many cilia of each sperm. In addition to their wonderful chromosomes, oocyte nuclei give a magnified view of various nuclear organelles1. The most obvious of these are the multiple nucleoli, which allow the oocyte to synthesize, in a very short time, enough ribosomes to set up several hundred thousand cells in the embryo. These nucleoli became important subjects for molecular biologists in the second half of the 1960s. Studying them changed concepts of the eukaryotic genome. They also served as model systems for working out a number of techniques, ranging from gene isolation to in situ hybridization. I began doctoral studies with Joe just as he was showing that oocyte nucleoli contain amplified copies of the genes for ribosomal RNA (see Fig. 1). The genes encoding ribosomal RNA are replicated to yield many copies that are not attached to the chromosome during oogenesis and then discarded before embryonic development. This was the first characterized example of cell-type-specific amplification of defined parts of the genome7,8. It was an exciting time to be in the Gall laboratory. Joe’s obvious delight in laboratory experimentation was contagious. It was also inspiring (and perhaps a little daunting) to a new graduate student to watch someone perform experiments so masterfully and pick up new techniques without missing a beat. Joe recognized that the pachytene oocyte, with its large cloud of amplified DNA, which had not yet been transformed into nucleoli, was the ideal test system for developing in situ hybridization9,10. The cell had a very large target for the only probe, ribosomal RNA, that could be obtained with sufficiently high specific activity and purity to do the experiment. (At that time, we were limited to in vivo labelling. By the next year, I was able to purify Escherichia coli RNA polymerase and use it to transcribe DNA fractionated on CsCl gradients. Clones – and kits – were not even dreamed of in those days.) Joe said, and I certainly agreed, that developing in situ hybridization was too much of a long shot for a thesis project, but we couldn’t refrain from talking about it and playing with it. When the long shot paid off, I was indeed lucky. It was a thesis project after all. The Gall laboratory in those days was a happy ecosystem. Following Joe’s knack of choosing the best biological system for any problem, people worked on many organisms, including Xenopus, Tetrahymena, Sciara, Rhynchosciara and Cecropia. When I showed by in situ hybridization that mouse satellite DNA was localized to heterochromatin and predicted that other satellites would also be located in heterochromatin11, Joe remembered that Drosophila virilis had trends in CELL BIOLOGY (Vol. 8) May 1998
Throughout his career, Joe has had a strong interest in the history of cell biology and has collected early books on cell biology. His selections of historically important and aesthetically pleasing illustrations from these books made the covers of the first 60 issues of Molecular Biology of the Cell collector’s items. Fortunately, those covers and the descriptions of each illustration have recently been collected in a book, Views of the Cell: A Pictorial History. It is entirely fitting that a scientist whose work is consistently at the forefront of the field should have a deep understanding of its history.
chromosomes that were nearly half heterochromatin. He and several students went on to show that 41% of the D. virilis genome is composed of three families of satellite DNA, each made up of remarkably homogeneous heptamer repeats12. Each fraction differs from the other two at one or two positions in the heptamer,
FIGURE 1 Autoradiograph of nuclei from a Xenopus laevis ovary preparation after in situ hybridization with 3H-labelled ribosomal RNA. Chromatin is visualized with magenta stain. The two most mature oocyte nuclei have caps of amplified genes for ribosomal RNA (rDNA) sitting over one side of the mass of chromosomes. The rDNA is heavily labelled by the radioactive probe. The other large nuclei are from less mature oocytes. These nuclei have smaller masses of hybridizing material, illustrating the progressive increase of extrachromosomal rDNA as oogenesis proceeds. The small, darkly stained nuclei are from somatic supporting cells of the ovary. Somatic nuclei show significant label over their rDNA only after much longer autoradiographic exposures because they contain only the diploid number of ribosomal genes.
209
pioneers ATAAACT, ACAAATT and ACAAACT (Ref. 13). The sequencing was done before the development of DNAsequencing techniques and was accomplished by cumbersome RNA sequencing, using in vitro transcripts of purified satellite DNA. Nevertheless, by picking the appropriate biological material, Joe had obtained the sequence of 41% of the D. virilis genome – many years before other genome projects began. His work also showed that these satellite DNAs are specifically underreplicated in polytene salivary gland cells12 and some other tissues14 while being overreplicated in ovaries. These cell-type-specific changes in amount of satellite DNA were the second examples of the dynamic nature of eukaryotic genomes. The resemblance between the multiple nucleoli of Tetrahymena macronuclei and those of amphibian oocytes suggested to Joe that genes encoding ribosomal RNA might be amplified independently of the chromosome in Tetrahymena15,16. He proved this was true, and his graduate student Kathleen Karrer showed that the amplified DNA existed as a linear molecule, with two coding regions arranged head-to-head17. A postdoctoral fellow, Elizabeth Blackburn, analysed the ends of this linear DNA and found that they consisted of perfect repeats of the hexanucleotide T4G2 (Ref. 18). This repeat has since become very famous because, after setting up her own research program, Liz, and others, have gone on to show that similar repeats form the telomeres of nearly every animal and plant19 (except, I note, Drosophila and Chironomus). Soon afterwards, Liz identified a new enzyme, telomerase, that adds the repeats to chromosome ends20. The success in studying the multiple nucleoli in amphibian oocytes in the late 1960s did not lead directly to explanations of the other structures in oocyte nuclei. Solving those structures and getting clues to their functions had to await the development of specific gene probes and monoclonal antibodies. With these tools now available, Joe is again studying oocyte nuclei, especially two major classes of non-nucleolar ribonucleoprotein particles (RNPs) found in these cells. He has shown that the most abundant class of RNPs, bodies smaller than the nucleoli, contain splicing factors – snRNPs and SR proteins. Because of their contents, he has named these bodies B snurposomes21. Members of the other major class of nuclear RNP organelles have been called spheres, although Joe has shown that they are equivalent to the coiled bodies in somatic cells22. It is intriguing that a few spheres are found at specific sites on the lampbrush chromosomes. Joe has shown that these sites are histone gene loci23,24 and that the spheres contain components necessary for forming the 3⬘ end of histone mRNA25 as well as components of other RNA-processing systems22. Joe’s studies of organelles in oocyte nuclei have contributed to the rapidly growing interest in intranuclear organelles in all cell types. Comparison of organelles from different cell types is very informative. For example, Joe has demonstrated similarities and differences between spheres in amphibian oocytes, Binnenkörper in insect oocytes, coiled bodies in somatic cells and the bodies that can be assembled
210
in vitro from Xenopus egg extracts. From this comparison, he has suggested that these organelles are composite structures containing components of several RNA-processing pathways and that the mix of components in each body might reflect the physiology of the cell26. We are beginning to get glimpses of how the eukaryotic nucleus works, and, in his seventieth year, Joe is still pioneering the field. References 1 Gall, J. G. (1954) J. Morph. 94, 283–351 2 Gall, J. G. and Callan, H. G. (1962) Proc. Natl. Acad. Sci. U. S. A. 48, 562–570 3 Gall, J. G. (1963) Nature 198, 36–38 4 Gall, J. G. (1967) J. Cell Biol. 32, 391–399 5 Gall, J. G. (1961) J. Biophys. Biochem. Cytol. 10, 163–193 6 Mizukami, I. and Gall, J. G. (1966) J. Cell Biol. 29, 97–111 7 Gall, J. G. (1968) Proc. Natl. Acad. Sci. U. S. A. 60, 553–560 8 Brown, D. D. and Dawid, I. B. (1968) Science 160, 272–280 9 Gall, J. G. and Pardue, M. L. (1969) Proc. Natl. Acad. Sci. U. S. A. 63, 378–383 10 Pardue, M. L. and Gall, J. G. (1969) Proc. Natl. Acad. Sci. U. S. A. 64, 600–604 11 Pardue, M. L. and Gall, J. G. (1970) Science 168, 1356–1358 12 Gall, J. G., Cohen, E. H. and Polan, M. L. (1971) Chromosoma 33, 319–344 13 Gall, J. G. and Atherton, D. D. (1974) J. Mol. Biol. 85, 633–664 14 Endow, S. A. and Gall, J. G. (1975) Chromosoma 50, 175–192 15 Gall, J. G. (1974) Proc. Natl. Acad. Sci. U. S. A. 71, 3078–3087 16 Yao, M-C. and Gall, J. G. (1977) Cell 12, 121–132 17 Karrer, K. M. and Gall, J. G. (1976) J. Mol. Biol. 104, 421–453 18 Blackburn, E. H. and Gall, J. G. (1978) J. Mol. Biol. 120, 33–53 19 Henderson, E. (1995) in Telomeres (Blackburn, E. H. and Greider, C. W., eds), pp. 11–34, Cold Spring Harbor Laboratory Press 20 Greider, C. W. and Blackburn, E. H. (1985) Cell 43, 405–413 21 Gall, J. G. (1991) Science 252, 1499–1500 22 Gall, J. G. et al. (1995) Dev. Genet. 16, 25–35 23 Gall, J. G. et al. (1981) Chromosoma 84, 159–171 24 Callan, H. G., Gall, J. G. and Murphy, C. (1991) Chromosoma 101, 245–251 25 Wu, C-H. H. and Gall, J. G. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 6257–6259 26 Bauer, D. W. and Gall, J. G. (1997) Mol. Biol. Cell 8, 73–82
AUTHORS’ CORRECTIONS The author of the December 1997 review ‘Expeditions to the pole: RNA localization in Xenopus and Drosophila’ (Trends Cell Biol. 7, 485–492) would like to clarify Fig. 1a (late pathway) by emphasizing that, during stages 1 and 2, Vg1 RNA is present throughout the cytoplasm of the Xenopus oocyte but is excluded from the perinuclear mitochondrial aggregates and mitochondrial cloud. This RNA distribution should have been represented by green shading of the cytoplasm, excluding the aggregates and cloud. The author of the April 1998 comment article ‘Atomic structures of tubulin and FtsZ’ (Trends Cell Biol. 8, 133–137) wishes to correct a mistake in Fig. 2b. The  strands in the C-terminal domain of -tubulin labelled in the figure as B7, B10, B8 and B9 should be labelled B9, B8, B10 and B7, from bottom to top. The authors apologize for any confusion caused.
trends in CELL BIOLOGY (Vol. 8) May 1998
Joseph Gall – pioneering nuclear biology Mary Lou Pardue By the mid-1960s, spectacularly successful studies had revealed much of the molecular biology of bacteria and viruses – how their genes are transcribed, translated and replicated. The next question was to what extent this information could be applied to multicellular organisms. What would be different about eukaryotic cells? Very few people had started to attack those questions when I began looking for a place to do graduate work in eukaryotic molecular biology. The name that kept coming up in my search was Joseph Gall (Boxes 1 and 2). Joe was pioneering eukaryotic molecular biology in those days, and he still is today. A look at Joe’s career shows that he has continually moved at the forefront of the field, asking important questions and finding unexpected answers. He has also been innovative in developing BOX 1 – JOE GALL Joseph Gall received a PhD in Zoology from Yale in 1952. He joined the faculty of the University of Minnesota, rising to the rank of Professor. In 1964, he became the Professor of Biology (later the Ross Granville Harrison professor) at Yale, and in 1983 he moved to the Carnegie Institution in Baltimore, where he is now American Cancer Society Professor of Developmental Genetics. His many honours include membership in the National Academy of Sciences, Accademia Nazionale dei Lincei, Rome, American Academy of Arts and Sciences, and the American Philosophical Society. He has been President of both the American Society for Cell Biology and the Society for Developmental Biology.
Mary Lou Pardue is in the Dept of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139, USA. E-mail: mlpardue @MIT.EDU
208
and adapting new techniques to address problems in cell biology. In each case, the new technique has been immediately used to solve an interesting question. From the beginning, one of the hallmarks of Joe’s work has been his ability to choose experimental materials that are uniquely suited for study of the problem at hand. A childhood passion for insect collecting gave him a broad knowledge of natural history. This basis allowed him to identify organisms or cell types with useful ‘oddities’ and then to exploit these to understand more-general problems. Joe has worked on a wide variety of organisms, including amphibia, fruit flies, protozoa and ferns. His PhD thesis research is a good example of astute choice of material. Although he studied with Donald Poulson, a Drosophila geneticist, Joe chose to work on the lampbrush chromosomes of amphibian oocytes1. These are the largest chromosomes known, and the nuclei are big enough to be isolated manually. In addition, lampbrush chromosomes come from a cell that is in the midst of laying down the groundwork of much of the embryo and therefore allow us to observe some very important genetic activities (see Fig. 1 on page 207). Today, when we can detect a single gene with a nucleic acid probe and identify individual proteins with monoclonal antibodies, it is hard to realize how important chromosome size was for the early studies. The first question was how DNA, RNA and protein, whose functions were just being elucidated in studies on prokaryotes, were related to the structures that cytologists had described in nuclei of eukaryotic cells. The tools available were blunt – stains with some sort of specificity, in vivo incorporation of radioactive precursors or selective digestion by enzymes. All these tools required significant amounts of material to permit detection. DNA, for example, could be detected by the Feulgen staining reaction, yet the reaction produced a puzzling result when used on lampbrush chromosomes. As expected from studies on other, less remarkable, chromosomes, the long axes of the chromosomes showed positive staining. By contrast, the loops that project from the axes and give the lampbrush chromosomes their name, did not stain with Feulgen1. Other chromosomes did not show detectable loops (at least in the stages of the cell cycle when they are most amenable to cytological analysis), so there were no other studies to compare with this result. We now know that the non-staining of the lampbrush loops is due to the decondensed nature of the DNA in these regions; more-powerful new stains, such as DAPI, can detect DNA in the loops. At the time, however, the result only emphasized important existing questions about the nature of the projecting lampbrush loops and about whether chromosomes are single intact molecules of DNA. Joe’s experiments answered both of these questions. His studies of uridine incorporation2 showed that RNA synthesis occurs on the lampbrush loops, thus demonstrating for the first time a site of cellular RNA synthesis. An elegant analysis of the kinetics of DNase digestion of lampbrush chromosomes showed that the chromosome is a single DNA molecule, with lesscondensed regions forming the lampbrush loops, where RNA synthesis occurs3. Joe also took advantage
Copyright © 1998 Elsevier Science Ltd. All rights reserved. 0962-8924/98/$19.00 PII: S0962-8924(98)01247-1
trends in CELL BIOLOGY (Vol. 8) May 1998
pioneers BOX 2 – JOE GALL AND THE HISTORY OF CELL BIOLOGY
of the ease of isolating oocyte nuclei to obtain a look at the nuclear envelope, and his electron-microscopic studies established the existence of the octagonally symmetrical nuclear pore complex4. Amphibian oocytes have no obvious advantages for studying centriole replication. Therefore, to study this subject, Joe turned to spermiogenesis in a snail, Viviparous5, and sperm formation in a fern, Marsilea6. In both cases, the centriole replicates multiple times to give rise to the basal bodies for the many cilia of each sperm. In addition to their wonderful chromosomes, oocyte nuclei give a magnified view of various nuclear organelles1. The most obvious of these are the multiple nucleoli, which allow the oocyte to synthesize, in a very short time, enough ribosomes to set up several hundred thousand cells in the embryo. These nucleoli became important subjects for molecular biologists in the second half of the 1960s. Studying them changed concepts of the eukaryotic genome. They also served as model systems for working out a number of techniques, ranging from gene isolation to in situ hybridization. I began doctoral studies with Joe just as he was showing that oocyte nucleoli contain amplified copies of the genes for ribosomal RNA (see Fig. 1). The genes encoding ribosomal RNA are replicated to yield many copies that are not attached to the chromosome during oogenesis and then discarded before embryonic development. This was the first characterized example of cell-type-specific amplification of defined parts of the genome7,8. It was an exciting time to be in the Gall laboratory. Joe’s obvious delight in laboratory experimentation was contagious. It was also inspiring (and perhaps a little daunting) to a new graduate student to watch someone perform experiments so masterfully and pick up new techniques without missing a beat. Joe recognized that the pachytene oocyte, with its large cloud of amplified DNA, which had not yet been transformed into nucleoli, was the ideal test system for developing in situ hybridization9,10. The cell had a very large target for the only probe, ribosomal RNA, that could be obtained with sufficiently high specific activity and purity to do the experiment. (At that time, we were limited to in vivo labelling. By the next year, I was able to purify Escherichia coli RNA polymerase and use it to transcribe DNA fractionated on CsCl gradients. Clones – and kits – were not even dreamed of in those days.) Joe said, and I certainly agreed, that developing in situ hybridization was too much of a long shot for a thesis project, but we couldn’t refrain from talking about it and playing with it. When the long shot paid off, I was indeed lucky. It was a thesis project after all. The Gall laboratory in those days was a happy ecosystem. Following Joe’s knack of choosing the best biological system for any problem, people worked on many organisms, including Xenopus, Tetrahymena, Sciara, Rhynchosciara and Cecropia. When I showed by in situ hybridization that mouse satellite DNA was localized to heterochromatin and predicted that other satellites would also be located in heterochromatin11, Joe remembered that Drosophila virilis had trends in CELL BIOLOGY (Vol. 8) May 1998
Throughout his career, Joe has had a strong interest in the history of cell biology and has collected early books on cell biology. His selections of historically important and aesthetically pleasing illustrations from these books made the covers of the first 60 issues of Molecular Biology of the Cell collector’s items. Fortunately, those covers and the descriptions of each illustration have recently been collected in a book, Views of the Cell: A Pictorial History. It is entirely fitting that a scientist whose work is consistently at the forefront of the field should have a deep understanding of its history.
chromosomes that were nearly half heterochromatin. He and several students went on to show that 41% of the D. virilis genome is composed of three families of satellite DNA, each made up of remarkably homogeneous heptamer repeats12. Each fraction differs from the other two at one or two positions in the heptamer,
FIGURE 1 Autoradiograph of nuclei from a Xenopus laevis ovary preparation after in situ hybridization with 3H-labelled ribosomal RNA. Chromatin is visualized with magenta stain. The two most mature oocyte nuclei have caps of amplified genes for ribosomal RNA (rDNA) sitting over one side of the mass of chromosomes. The rDNA is heavily labelled by the radioactive probe. The other large nuclei are from less mature oocytes. These nuclei have smaller masses of hybridizing material, illustrating the progressive increase of extrachromosomal rDNA as oogenesis proceeds. The small, darkly stained nuclei are from somatic supporting cells of the ovary. Somatic nuclei show significant label over their rDNA only after much longer autoradiographic exposures because they contain only the diploid number of ribosomal genes.
209
pioneers ATAAACT, ACAAATT and ACAAACT (Ref. 13). The sequencing was done before the development of DNAsequencing techniques and was accomplished by cumbersome RNA sequencing, using in vitro transcripts of purified satellite DNA. Nevertheless, by picking the appropriate biological material, Joe had obtained the sequence of 41% of the D. virilis genome – many years before other genome projects began. His work also showed that these satellite DNAs are specifically underreplicated in polytene salivary gland cells12 and some other tissues14 while being overreplicated in ovaries. These cell-type-specific changes in amount of satellite DNA were the second examples of the dynamic nature of eukaryotic genomes. The resemblance between the multiple nucleoli of Tetrahymena macronuclei and those of amphibian oocytes suggested to Joe that genes encoding ribosomal RNA might be amplified independently of the chromosome in Tetrahymena15,16. He proved this was true, and his graduate student Kathleen Karrer showed that the amplified DNA existed as a linear molecule, with two coding regions arranged head-to-head17. A postdoctoral fellow, Elizabeth Blackburn, analysed the ends of this linear DNA and found that they consisted of perfect repeats of the hexanucleotide T4G2 (Ref. 18). This repeat has since become very famous because, after setting up her own research program, Liz, and others, have gone on to show that similar repeats form the telomeres of nearly every animal and plant19 (except, I note, Drosophila and Chironomus). Soon afterwards, Liz identified a new enzyme, telomerase, that adds the repeats to chromosome ends20. The success in studying the multiple nucleoli in amphibian oocytes in the late 1960s did not lead directly to explanations of the other structures in oocyte nuclei. Solving those structures and getting clues to their functions had to await the development of specific gene probes and monoclonal antibodies. With these tools now available, Joe is again studying oocyte nuclei, especially two major classes of non-nucleolar ribonucleoprotein particles (RNPs) found in these cells. He has shown that the most abundant class of RNPs, bodies smaller than the nucleoli, contain splicing factors – snRNPs and SR proteins. Because of their contents, he has named these bodies B snurposomes21. Members of the other major class of nuclear RNP organelles have been called spheres, although Joe has shown that they are equivalent to the coiled bodies in somatic cells22. It is intriguing that a few spheres are found at specific sites on the lampbrush chromosomes. Joe has shown that these sites are histone gene loci23,24 and that the spheres contain components necessary for forming the 3⬘ end of histone mRNA25 as well as components of other RNA-processing systems22. Joe’s studies of organelles in oocyte nuclei have contributed to the rapidly growing interest in intranuclear organelles in all cell types. Comparison of organelles from different cell types is very informative. For example, Joe has demonstrated similarities and differences between spheres in amphibian oocytes, Binnenkörper in insect oocytes, coiled bodies in somatic cells and the bodies that can be assembled
210
in vitro from Xenopus egg extracts. From this comparison, he has suggested that these organelles are composite structures containing components of several RNA-processing pathways and that the mix of components in each body might reflect the physiology of the cell26. We are beginning to get glimpses of how the eukaryotic nucleus works, and, in his seventieth year, Joe is still pioneering the field. References 1 Gall, J. G. (1954) J. Morph. 94, 283–351 2 Gall, J. G. and Callan, H. G. (1962) Proc. Natl. Acad. Sci. U. S. A. 48, 562–570 3 Gall, J. G. (1963) Nature 198, 36–38 4 Gall, J. G. (1967) J. Cell Biol. 32, 391–399 5 Gall, J. G. (1961) J. Biophys. Biochem. Cytol. 10, 163–193 6 Mizukami, I. and Gall, J. G. (1966) J. Cell Biol. 29, 97–111 7 Gall, J. G. (1968) Proc. Natl. Acad. Sci. U. S. A. 60, 553–560 8 Brown, D. D. and Dawid, I. B. (1968) Science 160, 272–280 9 Gall, J. G. and Pardue, M. L. (1969) Proc. Natl. Acad. Sci. U. S. A. 63, 378–383 10 Pardue, M. L. and Gall, J. G. (1969) Proc. Natl. Acad. Sci. U. S. A. 64, 600–604 11 Pardue, M. L. and Gall, J. G. (1970) Science 168, 1356–1358 12 Gall, J. G., Cohen, E. H. and Polan, M. L. (1971) Chromosoma 33, 319–344 13 Gall, J. G. and Atherton, D. D. (1974) J. Mol. Biol. 85, 633–664 14 Endow, S. A. and Gall, J. G. (1975) Chromosoma 50, 175–192 15 Gall, J. G. (1974) Proc. Natl. Acad. Sci. U. S. A. 71, 3078–3087 16 Yao, M-C. and Gall, J. G. (1977) Cell 12, 121–132 17 Karrer, K. M. and Gall, J. G. (1976) J. Mol. Biol. 104, 421–453 18 Blackburn, E. H. and Gall, J. G. (1978) J. Mol. Biol. 120, 33–53 19 Henderson, E. (1995) in Telomeres (Blackburn, E. H. and Greider, C. W., eds), pp. 11–34, Cold Spring Harbor Laboratory Press 20 Greider, C. W. and Blackburn, E. H. (1985) Cell 43, 405–413 21 Gall, J. G. (1991) Science 252, 1499–1500 22 Gall, J. G. et al. (1995) Dev. Genet. 16, 25–35 23 Gall, J. G. et al. (1981) Chromosoma 84, 159–171 24 Callan, H. G., Gall, J. G. and Murphy, C. (1991) Chromosoma 101, 245–251 25 Wu, C-H. H. and Gall, J. G. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 6257–6259 26 Bauer, D. W. and Gall, J. G. (1997) Mol. Biol. Cell 8, 73–82
AUTHORS’ CORRECTIONS The author of the December 1997 review ‘Expeditions to the pole: RNA localization in Xenopus and Drosophila’ (Trends Cell Biol. 7, 485–492) would like to clarify Fig. 1a (late pathway) by emphasizing that, during stages 1 and 2, Vg1 RNA is present throughout the cytoplasm of the Xenopus oocyte but is excluded from the perinuclear mitochondrial aggregates and mitochondrial cloud. This RNA distribution should have been represented by green shading of the cytoplasm, excluding the aggregates and cloud. The author of the April 1998 comment article ‘Atomic structures of tubulin and FtsZ’ (Trends Cell Biol. 8, 133–137) wishes to correct a mistake in Fig. 2b. The  strands in the C-terminal domain of -tubulin labelled in the figure as B7, B10, B8 and B9 should be labelled B9, B8, B10 and B7, from bottom to top. The authors apologize for any confusion caused.
trends in CELL BIOLOGY (Vol. 8) May 1998