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The a mating type of locus of U. maydis specifies cell signalling components
headlines The a mating type locus of U. maydis specifies cell signalling components M. Bolker, M. Urban and R. Kahmann Cell 68, 441 4 5 0
A two-component regulatory system for self/nonself recognition in Ustilago maydis B. Gillissen, J. Bergemann, C. Sandmann, B. Schroeer, M. B61ker and R. Kahmann Cell 68, 647-657 In fungi, cell mating type is determined by expression of genes present at the mating-type locus MAT. The MAT genes of Saccharomyces cerevisiae and Neurospora encode transcriptional regulators that modulate expression of other genes, which, in turn, produce cell typespecific functions. The results presented in these papers demonstrate that the basidiomycete Ustilago maydis utilizes a novel mechanism to regulate cell type determination. U. maydis, the cause of corn smut, grows as a budding yeast when haploid. Two loci, a and b, control the cell's ability to mate and form a stable, filamentous heterokaryon. When cells carrying different matingtype alleles at each locus encounter one another, they fuse to form a
Signalling by sevenless protein tyrosine kinase is mimicked by Rasl activation M. E. Fortini, M. A. Simon and G. M. Rubin Nature 355, 559-561 There are now two cases that link activation of the Ras GTP-binding proteins to a pathway stimulated by a cell surface tyrosine kinase receptor. In 1990, Sternberg's group showed that formation of the vulva in Caenorhabditis elegans requires the let-23 gene, which encodes a tyrosine kinase, and let-60, which encodes a Ras protein. Loss of either let-23 or let-60 results in the absence of a vulva. Expression of a constitutively activated let-60 gene bypasses the requirement for let-23 and produces a multivulva phenotype. The second example comes from studies of the developing Drosophila eye. Differentiation of the R7 cell
heterokaryon. This heterokaryon then assumes a filamentous growth pattern, a requisite for infection and tumour production in corn plants. The paper by BOlker et al. shows that the a locus contains a pair of genes, mfal and mfa2. Each of these genes exists as two alleles that specify one or other mating type. However, rather than encoding regulatory proteins, these genes directly determine cell type: mfal encodes the secreted mating factor and mfa2 encodes the receptor that binds the mating factor produced by cells with the opposite allele. Thus, cell type in U. maydis is determined by critical structural genes at the mating-type locus. The paper by Gillissen et al. concerns the b locus, which is respon-
sible for filamentous growth and infectivity of heterokaryons following fusion. The authors demonstrate that the b locus also contains two genes, bE and bW, which both exist as multiple alleles. The bW gene, like the previously identified bE gene, encodes a protein with homeodomains. Only heterokaryons carrying different pairwise combinations of bE and bW alleles are able to produce filaments and infect plant cells. These results are consistent with the possibility that heterodimers formed between nonallelic bW and bE gene products directly regulate downstream genes necessary for the growth pattern alteration. Determining the targets of the regulatory complex is an exciting challenge for future research.
requires the tyrosine kinase receptor encoded by sevenless (sev). This tyrosine kinase is stimulated by the protein ligand encoded by bride of sevenless (boss). A genetic screen for mutants that enhance the effects of sev mutations has revealed the involvement of the rasl gene in R7 cell differentiation. This paper demonstrates the functional link between the products of rasl and sev. Expression of an activated rasl gene, encoding the mutant Rasl van2 protein, rescued null mutants of both
sev and boss and resulted in the generation of supernumerary R7 cells. This latest result suggests that interaction between Ras and tyrosine kinases may be generally important in signal transduction. Interestingly, the same genetic screen that identified the rasl gene revealed other genes involved in signalling by sev, one of which (Son ofsevenless, Sos) is a homoIogue of the Saccharomyces cerevisiae CDC25 gene, which encodes a guanine nucleotide exchange factor. The Sos protein may thus regulate Rasl.
TRENDS IN CELL BIOLOGYVOL. 2 MAY 1992
,,,
HEADLINES
The HEADLINES section.0f Trends in CetiBiology is intended to draw attenti0n to a selection Of research papers of irnportanceto ce f biology that have been published ii~ the last ~few monthS. HEADEiNES are Contributed regularly bY a panel of research scientists appoi.n~ed by the Editor: Scientists interested in joining the panel shou!d contact the Editor, Trends in Celt Biology; Elsevier Trends Ioumals, 68 Hills Road; Cambridge, U K CB2:1LA. 1 33
A two-component regulatory system for self/nonself recognition in Ustilago maydis B. Gillissen, J. Bergemann, C. Sandmann, B. Schroeer, M. B61ker and R. Kahmann Cell 68, 647-657 In fungi, cell mating type is determined by expression of genes present at the mating-type locus MAT. The MAT genes of Saccharomyces cerevisiae and Neurospora encode transcriptional regulators that modulate expression of other genes, which, in turn, produce cell typespecific functions. The results presented in these papers demonstrate that the basidiomycete Ustilago maydis utilizes a novel mechanism to regulate cell type determination. U. maydis, the cause of corn smut, grows as a budding yeast when haploid. Two loci, a and b, control the cell's ability to mate and form a stable, filamentous heterokaryon. When cells carrying different matingtype alleles at each locus encounter one another, they fuse to form a
Signalling by sevenless protein tyrosine kinase is mimicked by Rasl activation M. E. Fortini, M. A. Simon and G. M. Rubin Nature 355, 559-561 There are now two cases that link activation of the Ras GTP-binding proteins to a pathway stimulated by a cell surface tyrosine kinase receptor. In 1990, Sternberg's group showed that formation of the vulva in Caenorhabditis elegans requires the let-23 gene, which encodes a tyrosine kinase, and let-60, which encodes a Ras protein. Loss of either let-23 or let-60 results in the absence of a vulva. Expression of a constitutively activated let-60 gene bypasses the requirement for let-23 and produces a multivulva phenotype. The second example comes from studies of the developing Drosophila eye. Differentiation of the R7 cell
heterokaryon. This heterokaryon then assumes a filamentous growth pattern, a requisite for infection and tumour production in corn plants. The paper by BOlker et al. shows that the a locus contains a pair of genes, mfal and mfa2. Each of these genes exists as two alleles that specify one or other mating type. However, rather than encoding regulatory proteins, these genes directly determine cell type: mfal encodes the secreted mating factor and mfa2 encodes the receptor that binds the mating factor produced by cells with the opposite allele. Thus, cell type in U. maydis is determined by critical structural genes at the mating-type locus. The paper by Gillissen et al. concerns the b locus, which is respon-
sible for filamentous growth and infectivity of heterokaryons following fusion. The authors demonstrate that the b locus also contains two genes, bE and bW, which both exist as multiple alleles. The bW gene, like the previously identified bE gene, encodes a protein with homeodomains. Only heterokaryons carrying different pairwise combinations of bE and bW alleles are able to produce filaments and infect plant cells. These results are consistent with the possibility that heterodimers formed between nonallelic bW and bE gene products directly regulate downstream genes necessary for the growth pattern alteration. Determining the targets of the regulatory complex is an exciting challenge for future research.
requires the tyrosine kinase receptor encoded by sevenless (sev). This tyrosine kinase is stimulated by the protein ligand encoded by bride of sevenless (boss). A genetic screen for mutants that enhance the effects of sev mutations has revealed the involvement of the rasl gene in R7 cell differentiation. This paper demonstrates the functional link between the products of rasl and sev. Expression of an activated rasl gene, encoding the mutant Rasl van2 protein, rescued null mutants of both
sev and boss and resulted in the generation of supernumerary R7 cells. This latest result suggests that interaction between Ras and tyrosine kinases may be generally important in signal transduction. Interestingly, the same genetic screen that identified the rasl gene revealed other genes involved in signalling by sev, one of which (Son ofsevenless, Sos) is a homoIogue of the Saccharomyces cerevisiae CDC25 gene, which encodes a guanine nucleotide exchange factor. The Sos protein may thus regulate Rasl.
TRENDS IN CELL BIOLOGYVOL. 2 MAY 1992
,,,
HEADLINES
The HEADLINES section.0f Trends in CetiBiology is intended to draw attenti0n to a selection Of research papers of irnportanceto ce f biology that have been published ii~ the last ~few monthS. HEADEiNES are Contributed regularly bY a panel of research scientists appoi.n~ed by the Editor: Scientists interested in joining the panel shou!d contact the Editor, Trends in Celt Biology; Elsevier Trends Ioumals, 68 Hills Road; Cambridge, U K CB2:1LA. 1 33