Ribonucleoprotein particles and the maturation of eukaryote mRNA

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the different organs. Since the lipase 19 Hfilsmann, W. C., Oerlemans, M. C. and Jansen, 25 Jansen, H. (1979) in Edition des Colloques de H. (1980) Biochim. Biophys. Acta 618,364-369 I'INSERM: Obesity - Cellular and Molecular activity of the liver is generally much higher 20 Jansen. H., Oerlemans, M. C. and Hfilsmann, Aspects (Ailhaud, G., ed.), vol. 87, pp. 137than that of adrenals and ovaries the W. C. (1977) Biochem. Biophys. Res. Commun. 147 activity of the liver will principally deter77,861-867 26 Brown. M. S., Kovanen, P. T. and Goldstein, J. L. mine not only the amounts of high density 21 Jansen, H., Van Tol, A. and Hfilsmann, W. C. (1979) Recent Prog. Horm. Res. 352, 15-256 (1980) Biochem. Biophys. Res. Commun. 92, 27 Andersen, J. M. and Dietschy, J. M. (1978) lipoprotein cholesterol and phospholipid in 53-59 J. Biol. Chem. 253, 9024-9032 the serum but also other aspects of the 22 Kuusi, T., Kinnunen, P. K. J. and Nikkilfi, E. A. 28 Gwynne, J. T., Mahafee, D., Brewer, H. B. and metabolism of high density lipoprotein. (1979) FEBS Lett. 1(14,384-388 Ney, R. L. (1976) Proc. Natl. Acad. Sci. U.S.A. Lipoprotein lipase plays a role in the chan- 23 Rothblat, G. H., Buckko, M. K. and Kritchevsky, 73, 4329-4333 nelling of serum triglycerides to different D. (1968) Biochim. Biophys. Acta 164, 327-338 29 Astwood, E. B. (1939) Am. J. Physiol. 162, 126-132 organs 3. Our evidence suggests that liver 24 Applebaum, D., Goldberg, A. P., Hazzard, W. R., Sherard, D. J., Brunzell, J. D., Huttunen, J. K., 30 Everett, J. W. (1947) Endocrinology 41,364-377 lipase behaves in an analogous fashion to Nikkil~i, E. A. and Ehnholm, C. (1979) Metabol31 Morris, M. D. and Chaikoff, I. L. (1959) J. Biol. channel phospholipids and cholesterol into ism 28,917-924 Chem. 234, 1095-1097 the liver, and there appear to be comparable enzymes in several other organs, such as adrenals and ovaries. Many questions remain to be answered: (a) Is the proposed pathway related to other mechanisms of HDL-cholesterol uptake in liver and steroid hormoneJohn T. Knowler and Andrew F. Wilks producing organs? (b) What is the preferred in vivo high density lipoprotein substrate for liver lipase? (c) Are specific apolipoproteins (e.g. apolipoprotein E) The proteins of nudear ribonucleoprotein partides, together with low-molecular-weight nuclear RNA species may be responsible for the controlled maturation of mRNA. involved or should the particle first be partly depleted from C-apolipoproteins? It has long been suspected that cytoplasmic m R N A involves excision of the intron (d) Finally, how important is the enzyme in m R N A of eukaryote cells is the product of transcripts and the ligation of the coding the overall regulation of steroidogenesis? maturation from longer precursors in a portions of the message. process analogous to the maturation of the These splicing reactions, together with References structural RNAs of the ribosome. The other post-transcriptional modifications 1 Hahn, D. F. (1943)Science 98, 19-20 prime candidate for such a precursor has such as 'capping' and polyadenylation must 2 Vogel, W. C. and Zieve, L. (1964)J. Lipid Res. 5, always been the so-called heterogeneous involve enzymes which are likely to be 177-183 3 Robinson, D. S. (1970) Comp. Biochem. Physiol. nuclear RNA (hnRNA) which is transclosely associated with the RNA. Further18, 51-116 cribed and localized in the nucleus and more, it is likely that eukaryotes will use 4 LaRosa, J. C., Levy, R. I., Windmueller, H. G. exhibits a diversity of size which amply these events as a point at which control of and Fredrickson, D. S. (1972) J. Lipid Res. 13, earns it its name. gene expression might be exerted. It is for 356-363 Over the years the largely circumstantial these reasons that the proteins known for 5 Augustin, J. and Greten, H. (1979) Prog. Biochem. Pharmacol. 15, 5-40 evidence relating hnRNA and mRNA has many years to be associated with nuclear 6 Glomset, J. A. (1979 ) Prog. Biochem. Pharmacol. become much firmer. Initial ambiguous RNA are attracting increasing attention. 15,41-66 kinetic evidence was strengthened by the 7 Eisenberg, S. (1979) Prog. Biochem. Pharmacol. finding that both types of molecules often Heterogeneous ribonucleoprotein 15,139-165 particles 8 Jansen, H., Van Zuylen-Van Wiggen, A. and possess 3' polyadenosine tails and modHiilsmann, W. C. (1973) Biochem. Biophys. Res. ified 5' termini known as 'caps'. The greatFrom a very early ~stage in its transcripCommun. 55, 30-37 est problem with the precursor-product tion, hnRNA exists as ribonucleoprotein 9 Krauss, R. M., Windmueller, H. G., Levy, R. I. hypothesis was always the several fold difextractable as ribonucleoprotein particles and Fredrickson, D. S. (1973) J. Lipid Res. 13, ference in size between the two classes of (hnRNP). Electron microscope studies 356-363 molecules. Rationalization of this difficulty reveal that protein associates with hnRNA 10 Jansen, H. and Hiilsmann, W. C. (1975)Biochim. Biophys. Acta 398, 337-346 has come with the realization that the genes while it is still being transcribed 2, and free 11 Jansen, H., Kalkman, C., Birkenh~iger, J. C. and for proteins often contain inserts of nonhnRNA has not been detected in the cell. Hiilsmann, W. C. (1980) FEBS Lett. 112, 30-34 coding material (introns) which often con- The methods employed for the isolation of 12 Thomas, J., Debeer, L. J. and Mannaerts, P. G. siderably exceed the length of the coding hnRNP have been reviewed by Van Ven(1978) Biochem. J. 172, 177-179 rooij and Janssen ~. Basically they involve 13 Jansen, H., Van Berkel, Th. J. C. and Hiilsmann, portion (exons). By using radioactive comW. C. (1978)Biochem. Biophys. Res. Commun. plementary DNA copies of m R N A to hy- extraction of particles from purified nuclei, 85,148-152 bridize to nuclear RNA, it has recently or the fractionation of the products of nuc14 Sundaram, G. S., Shakir, K. M. M., Barnes, G. and been possible to demonstrate that m R N A lear lysis. Characteristically, the particles Margolis, S. (1978) J. Biol. Chem. 253, is derived from longer precursors. Furthconsist of approximately 20% RNA and 7703-7710 15 Jansen, H., Kalkman, C., Zonneveld, A. J. and ermore, the great length of the precursor 80% protein and, with adequate preHiilsmann, W. C. (1979) FEBS Lett. 98,299-302 RNA molecules is due, in large measure, to cautions, they can be isolated as polymers 16 Kuusi, T., Nikkil~i, E. A., Virtanen, I. and Kinnu- the fact that they contain transcripts of arranged like beads on a string of hnRNA. nen, P. K. J. (1979) Biochem. J. 181,245-246 introns 1. Maturation of the precursor into The RNA between the beads is, however, 17 Kinnunen, P. K. J. and Ehnholm, C. (1976) FEBS very vulnerable to attack by ribonuclease Lett. 65,354-357 John. T. Knowler and Andrew F. Wilks are at the leaving monomer particles which sediment 18 Waite, M. and Sisson, P. (1974)J. Biol. Chem. Department o f Biochemistry, University o f Glasgow, 249, 6401-6405 Glasgow G12 8QQ, U.K. at approximately 30--40 S. These have a

Ribonucleoprotein particles and the maturation of eukaryote mRNA

© Elsevier/North-Holland Biomedical Press 1980

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buoyant density of 1.40g cm -3 in CsCI, and contain RNA of 5-10 S in size. The heterogeneity which the monomer particles exhibit in density gradient centrifugation may reflect differences in the size of residual RNA left as tails on the particles. However, some workers have claimed to detect several distinct peaks on buoyant density gradients and the possibilily of different populations of particles cannot be excluded 4. Studies on the structure of hnRNP have not led to a clear picture of the interrelationship of protein and RNA. An early modeP placed the RNA around the particle in a manner analogous to the way in which DNA is thought to coil around the nucleosome. Other models 3,8 place the RNA coiled within the protein of the particle and partially protected between particles by other proteins. It is clear that the protein component does partially protect the RNA from nuclease attack a:ad that sequential removal of protein with salt renders the RNA more vulnerable. However, such findings can be accommodated by either model and, in the absence of more precise data, it appears poiatless to speculate further on the inter-relationship of the molecular species within the particle. It is also impossible to say much about the role that the intact particle may play in protecting specific sequences of hnRNA from nuclease attack. It is conceivable, for instance, that particles might initially form only on those portions of hnRNA which are to be conserved, leaving the transcripts of the introns without protection. However, the regular spacing of particles on hnRNA 2 compared with the !irregular spacing of introns argues against this possibility. A more rewarding approach to the study of the role of hnRNP particles in m R N A maturation ties in a study of the protein TABLE I. Enzyme activities detected in hnRNP particles Enzyme activity

Source

Ref.

Poly-A synthetase

Rat liver

11

'Capping' enzymes

Rat liver

12

Ribonuclease

Rat liver

13,21

Double-stranded RNA specific RNAse HeLa cells

14, 21

Protein kinase

Rat liver

10,15

Protein kinase

HeLa cells

16

Phosphoprotein phosphate

HeLa cells

22

species themselves. Considerable controversy has existed over the precise number of proteins which are associated with hnRNP particles. The discrepancies found in the published reports and their probable causes have been reviewed elsewher& "8. On balance, it appears that variations are largely due to differences in isolation technique causing loss of proteins, or the adventitous binding of protein not normally associated with the particles. As in studies of ribosomal proteins, the latter possibility is a particularly difficult one to exclude. The observed differences in the protein components reported by different workers using different systems and different tissues reflect the minor components. Virtually all investigators are agreed that the dominant group of proteins, collectively known as core proteins, are at least three and probably four species with molecular weights of between 30,000 and 45,000. It is widely assumed that these polypeptides are the structural proteins of the particle. They appear to be universally present and, in parallel with other nucleic acid binding proteins, i.e. histones and ribosomal proteins, they are basic 7 and have a low turnover rate 8. An abundance of glycine in their composition has led Lestourgeon 9 to suggest that they contain a high percentage of /3-sheet in their structure, and as such might be admirably suited for intercalating with double-stranded portions of RNA. On two-dimensional polyacrylamide gel fractionation systems, these major proteins exhibit charge heterogeneity 7,~° and, in at least one case, this appears to result from phosphorylation 1°. Such modification might permit variation in the degree of interaction between the protein and the RNA in the same way that modification of histones or protamines affects their association with DNA. Table I lists some of the enzyme activities that have been detected in hnRNP particles. It can be seen that a number of the enzymes which are associated with the post-transcriptional modification of hnRNA have been detected. These include an Mn2+-dependent poly-A polymerase" and a 5' capping enzymelL Nuclease activities are also present TM and these include an activity dependent on double-stranded RNA x4. Such an activity may have particular relevance with respect to the role of snRNA in hRNA maturation to be discussed below. Of considerable interest from the point of view of the possible control of messenger RNA maturation by hnRNP proteins is the finding that the particles possess protein

kinase activity which is capable of phosphorylating several hnRNP particle proteins 1°,15,16. Recent observations in this laboratory TM reveal that the activity of the kinase(s) is considerably enhanced by cyclic AMP or polyamines, and that the pattern of liver hnRNP protein phosphorylation observed on two-dimensional gel fractionations is dramatically altered following adrenalectomy, and returned to normal by the administration of glucocorticoids. Such effects may be related to the known role of glucocorticoids in increasing the active amounts of mRNAs coding for liver gluconeogenic enzymes, and in stimulating tissue hypertrophy. Methylation of arginine residues of a number of hnRNP particle proteins has been reported in HeLa cells9 and rat liveP 5. Amino acid analysis revealed the unusual derivative N%N%-dimethylarginine as a component of core proteins in both systems. The methylated proteins from HeLa hnRNP show immunological identity with a P h y s a r u m RNA-binding protein which also contains the dimethylarginine derivative. A significant function for the unusual residue and the protein on which it occurs is suggested by the apparent evolutionary conservation. Lestourgeon and coworkers 9 have suggested that dimethylarginine is only found in significant amounts in RNP structures and may be involved in RNA-protein interaction.

SnRNP particles For more than a decade it has been known that low-molecular-weight RNA species occur in the nucleus. These include tRNA and 5.8 S RNA destined for transport to the cytoplasm, and a group of RNA species collectively known as snRNA (small nuclear RNA), found only in the nucleus. There are six major species designated U1 to U6, some of which have minor structural variants. Of these, U3 and its variants occur only in the nucleolus a7while the others are nucleoplasmic in origin. They contain 90-220 bases and are notable for their modified nucleotides which include a 'cap-like' 2,2,7-trimethylguanine at the 5' end, together with internal methylated residues and psuedouridine. Of the nucleoplasmic species, U1 and U2 have been sequenced. Sn-RNA has frequently been detected in ribonucleoprotein particles isolated from nuclei, and Sekeris and Niesing TM included them in a model of hnRNP particle structure. However, recent striking evidence has indicated that they exist in separate particles and may well play an important role in hnRNA maturation. Joan Steitz and

270 her colleagues TM have demonstrated that some patients with systemic lupus erythematosis, an autoimmune rheumatic disease, produce antibodies which specifically complex with what they call snRNP particles. These particles contain seven polypeptides and the five nucleoplasmic snRNAs. Different antisera precipitate different subsets of particles which vary in their snRNA composition, but always contain the same seven protein components. The evidence suggests that each snRNA exists as a separate particle with an indistinguishable set of seven polypeptides. The polypeptides have molecular weights ranging from 12,000 to 35,000 and are unrelated either to hnRNP core proteins or to histones. More recently, Steitz and co-workers 2° have put forward a number of findings which imply that snRNP may play a crucial role in the maturation of hnRNA. Firstly, the components of the complexes have been strongly conserved through evolution. The antiserum is able to precipitate complexes from man, mouse, chicken, frog, sea urchin and an insect. Since they demonstrate that the antigen is the protein of the snRNP, their data imply a strong similarity in the protein components of the particles from these species. At least some of the snRNAs have also been strongly conserved. U1 RNA of the HeLa cells has the same sequence as U 1a of mouse and rat but U1 of an insect differs in sequence. The antisera from systemic lupus erythematosis patients were not able to precipitate particles from tobacco, yeast, a slime mould orE. coli. It may be significant that yeast cells are apparently unable to splice globin mRNA out of its precursor. Further findings implying that snRNPs are involved in hnRNA maturation are that they are most abundant in metabolically active cells and, in these circumstances, no longer sediment at 10 S but are found with hnRNP at 30 S. Steitz and co-workers 2° speculate that the shift may be due to the attachment of snRNP to hnRNP. In support of this hypothesis is the demonstration that the 5' end of the most abundant snRNA, U1, contains a sequence which shows extensive base pairing with the intron-exon boundry sequences of hnRNA. A number of studies have been performed on the boundaries between introns and exons of genes or their transcripts, as it is in this region that recognition by splicing enzymes might be expected to occur. Initial findings reveal that the boundary regions are not identical. Nevertheless, they do show considerable homology and several

TIBS - October 1980

groups have proposed 'consensus sequences' from which any of the known sequences differ by only a few nucleotides. Such a consensus sequence derived from 26 5' ends of introns and 31 3' ends of introns reveals for instance that the-dinucleotide G U is immediately adjacent to the splice point of all but one 5' end while A G is immediately adjacent to the splice point at all but one 3' end 2°. The 5' end of the snRNA U1, shows extensive complementarity to the consensus sequence. Nucleotides 3 to 8 exactly match the consensus 5' end of the intron, and nucleotides 9 to 11 exactly match the 3' end. Some pairing is also possible with nucleotides 12-20. Here then is a mechanism by which splicing may be made mechanically feasible. If the 5' end of U1 base pairs with both ends of the intron, the remainder of the intron will loop out and the two exons will be aligned for ligation (Fig. 1).

Supporting evidence for this scheme comes from the finding that if the 5' end of U1 is removed, as happens to a percentage of molecules during isolation, the snRNP containing the incomplete RNA always sediments at 10 S and never associates with 30 S particles. Steitz and co-workers are aware however that the consensus sequence is unlikely to provide sufficient com~,plementarity for the specificity of splicing. IIt is likely that proteins, possibly those of both snRNP and hnRNP, are also involved. The situation would thus be analogous to the binding of the 5' end of prokaryote mRNAs to 16 S RNA where there is some complementarity between the two nucleic acid molecules, but ribosomal proteins are also involved. Steitz and her colleagues also suggest that the intact snRNP could function as an RNAase with the RNA providing much of the binding site and the proteins effecting the catalysis. They draw attention to the precedent of E. coli EXON TRANSCRIPT

INTRON TRANSCRIPT

EXON I ,, - ~O / " TRANSCRI PT \GOb'A~ • 11s

/

.....

3'

5' cgncensus sequence

3' concensus sequence'

hnR m~'2ZGpPPArnu.%.

z-

%ooo

5' end of

GO

looped out 1 intron transcript

%%

l/

f

"3'

s~

\

/

/

\

!

~ ,/

19 I I

\

\

/

I

3' end of U I RNA

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c4AO6 G ~D~intron-exon junction 5'end of hnRNA j

3'end of hnRNA

Fig. 1. Possible base pairing between the intron~xon functions o f h n R N A and UI RNA. Y indicates pyrimidines and X a variable base.

271

TIBS - October 1980

RNAase P which contains a small RNA complexed with several proteins. As already stated, an RNAase activity dependent on double-stranded RNA has already been demonstrated in hnRNP particles. An understanding of the maturation of eukaryote mRNA has only just begun but it is certain that we can look forward to a quickening pace in the future. References 1 Ryffel, G. U., Wyler, T., Muellener, D. B. and Weber, R. (1980) Cell 19, 53-61 2 Miller, O. L. and Hamkalo, B. A. (1972) Int. Rev. Cytol. 33, 1-25 3 Van Venrooij, W. J. and Janssen, D. B. (1978) Mol. Biol. Rep. 4, 3-8 4 Houssais, J. F. (1975) FEBS Lett. 56, 341-347

5 Samarina, O. P., Lukanidin, E. M., Molnar, J. and Georgiev, G. P. (1968) J. Mol. Biol. 33,251-263 6 Heinrich, P. C., Gross, V., Northemann, W. and Scheurlen, M. (1978) Rev. Physiol. Biochem. Pharmacol. 81,102-134 7 Suria, D. and Liew, C. C. (1979) Can. J. Biochem. 57, 32-42 8 Martin, T., Jones, R. and Billings, P. (1979) Mol. Biol. Rep. 5, 37-42 9 Lestourgeon, W. M., Beyer, A. L., Christensen, M. E., Walker, B. W., Poupore, S. M. and Daniels, L. P. (1977) Cold Spring Harbor Quant. Syrup. Biol. 42, 885-898 10 Wilks, A. F. and Knowler, J. T. (submitted) 11 Niessing, J., and Sekeris, C. E. (1973) Nature (London) New Biol. 243, 9-12 12 Bajszar, G., Szabo, G., Simincsits, A. and Molnar, J. (1978) Mol. Biol. Rep. 4, 93-96 13 Niessing, J. and Sekeris, C. E. (1970) Biochim. Biophys. Acta 209, 484-492

14 Rech, J., Brunel, C. and Jeanteur, P. L. (1979) Biochem. Biophys. Res. Commun. 88, 422-427 15 Karn, J., Vidati, G., Boffa, L. C. and Allfrey, V. G. (1977)J. Biol. Chem. 252, 7307-7322 16 Blanchard, J.-M., Brunel, C. and Jeanteur, P. (1977) Eur. J. Biochem. 79, 117-131 17 Zieve, G. and Penman, S. (1976) Cell 8, 19-31 18 Sekeris, C. E. and Niessing, J. (1975) Biochem. Biophys. Res. Commun. 62, 642-650 19 Lerner, M. R. and Steitz, J. A. (1979)Proc. Natl. Acad. Sci. U.S.A. 76, 5495-5499 20 Lerner, M. R., Boyle, J. A., Mount, S. M., Wolin, S. L. and Steitz, J. A. (1980) Nature (London) 283,220-224 21 Molnar, J., Bajsz~ir, G., Marczinovits, I. and Szabo, G. (1978) Mol. Biol. Rep. 4, 157-161 22 Periasamy, M., Brunel, L., Blanchard, J.-M. and Jeanteur, P. (1977) Biochem. Biophys. Res. Commun. 79, 1077-1083

cal model for glyoxysome formation is based on that initially proposed for mammalian peroxisomes, which we assume should be mechanistically similar. This J. Michael b3rd and Lynne M. Roberts model implies that glyoxysomes are derived directly from the endoplasmic Proteins that span the glyoxysomal membrane are synthesized and inserted into the mem- reticulum (ER) 3 as budding outgrowths; brane at the same time. Recent evidence indicates that enzymic proteins o f the glyoxysomal the glyoxysomal membrane is thus formed by a process of membrane flow. The enmatrix may cross a membrane after they have been synthesized. zymic proteins of the glyoxysomal matrix A glyoxysome is a specialized form of This compartmentation of enzymes are thought to be made on membraneperoxisome (microbody) that is found in accounts for this tissue's remarkably ef- bound ribosomes (rough ER) and the certain plant tissues. They occur in the ficient gluconeogenesis from stored fats; nascent proteins are segregated into the endosperm or cotyledonary cell.,; of fat- acetyl-SCoA generated during B-oxidation intracisternal space. The conceptual feastoring seeds and are only present in sig- avoids the oxidative decarboxylations of tures of this pathway are based on the nificant numbers during the early stages of the mitochondrial citric acid cycle and 75 % established intracellular route for the growth after germination when rapid of the fatty acid carbon is ultimately re- synthesis and segregation of secretory proteins so elegantly elucidated by Palade and gluconeogenesis from stored triglycerides covered in sucrose ~. Glyoxysomes are not found in dry seeds. his co-workers4. Indeed, it was generally is the dominant metabolic process. Glyoxysomes were first isolated and characterized During the germination of castor bean assumed that all proteins which must cross from the endosperm tissue of germinating seeds at 30°C, glyoxysomes and their con- an intracellular membrane before reaching seedlings of castor bean (Ricin,~ com- stituent enzymes are rapidly synthesized de their target organelle are synthesized on munis). Their morphological and biochem- novo during the first 5 days of growth. membrane-bound ribosomes - an assumpical properties are similar to those of After this time they disappear equally tion now known to be false for all mammalian peroxisomes as described by rapidly as fat utilization is completed and mitochondrial and chloroplast proteins de Duve and his colleagues1: they are the endosperm cell senesces as the seed- that are synthesized in the cytoplasm and roughly spherical organelles aboul 1 p.m in lings become able to photosynthesize. then migrate to the organelle 5'e. Electron diameter comprising a protein matrix that Because of the magnitude of gluco- micrographs prepared from a wide range of sometimes contains a crystalline core, and neogenesis from fats (triglycerides account eukaryotic cells show a characteristic close are bounded by a single membrane with an for some 60% of the dry weight of castor association, occasionally direct membrane enzymic complement which includes catal- bean seeds), at the peak of their activity continuity, between peroxisomes or glyoxysomes contain 20% of the total par- glyoxysomes and the ER s. This evidence is ase, uricase and a-hydroxy acid oxidase. The glyoxysomes of fatty seeds play a ticulate protein present in the endosperm consistent with the biochemical evidence that microbody enzymes cross the memwell-defined and necessary role in cell. Germinating fatty seedlings are useful brane as they are being synthesized. gluconeogenesis from triglycerides in conMicrobody enzymes that are discharged trast to their uncertain role in most for studying the assembly of glyoxysomes eukaryotic cells. In castor beans the because in these seedlings large numbers of into the lumen of the ER as they are synenzymes catalysing the activation and glyoxysomes are formed rapidly and these thesized would presumably be sorted to separate them from proteins destined for fl-oxidation of long-chain, fatty acids and can be easily isolated. secretion or another cellular compartment. the complete glyoxylate cycle (Fig. 1) are However, recent experimental evidence, located exclusively in the glyoxysomes. Glyoxysome formation - the 'classical' model discussed below, indicates that proteins Glyoxysomes depend upon biosynthetic destined for the microbody matrix might be J. Michael Lord and Lynne M. Roberts are at the events elsewhere in the cell to provide their translocated across the membrane only School of Biological Sciences, University of Bradford, membrane and matrix proteins. The classi- after their synthesis has been completed. Brad]brd, W. Yorks BD7 1DP, U.K.

Formation of glyoxysomes

© Elsevier/North-HollandBiomedicalPress 1980