Genetic Control and Gene Expression During Meiosis and Sporulation in Saccharomyces cerevisiae

SSG1, a gene encoding a sporulation-specific 1,3-beta-glucanase in Saccharomyces cerevisiae

In Saccharomyces cerevisiae, the meiotic process is accompanied by a large increase in 1,3-beta-glucan-degradative activity. The molecular cloning of the gene (SSG1) encoding a sporulation-specific exo-1,3-beta-glucanase was achieved by screening a genomic library with a DNA probe obtained by polymerase chain reaction amplification using synthetic oligonucleotides designed according to the nucleotide sequence predicted from the amino-terminal region of the purified protein. DNA sequencing indicates that the SSG1 gene specifies a 445-amino-acid polypeptide (calculated molecular mass, 51.8 kDa) showing extensive similarity to the extracellular exo-1,3-beta-glucanases encoded by the EXG1 gene (C. R. Vazquez de Aldana, J. Correa, P. San Segundo, A. Bueno, A. R. Nebreda, E. Mendez, and F. del Rey, Gene 97:173-182, 1991). The N-terminal domain of the putative precursor is a very hydrophobic segment with structural features resembling those of signal peptides of secreted proteins. Northern (RNA) analysis reveals a unique SSG1-specific transcript, 1.7 kb long, which can be detected only in sporulating diploids (MATa/MAT alpha) but does not appear in vegetatively growing cells or in nonsporulating diploids (MAT alpha/MAT alpha) when incubated under nitrogen starvation conditions. The meiotic time course of SSG1 induction indicates that the gene is transcribed only in the late stages of the process, beginning at the time of meiosis I and reaching a maximum during spore formation. Homozygous ssg1/ssg1 mutant diploids are able to complete sporulation, although with a significant delay in the appearance of mature asci.

Isolation of two developmentally regulated genes involved in spore wall maturation in Saccharomyces cerevisiae

During sporulation of Saccharomyces cerevisiae, the four haploid nuclei generated by meiosis are encapsulated within multilayered spore walls. Taking advantage of the natural fluorescence imparted to yeast spores by the presence of a dityrosine-containing macromolecule in the spore wall, we identified and cloned two genes, termed DIT1 and DIT2, which are required for spore wall maturation. Mutation of these genes has no effect on the efficiency of spore formation or spore viability. The mutant spores, however, fail to accumulate the spore wall-specific dityrosine and lack the outermost layer of the spore wall. The absence of this cross-linked surface layer reduces the resistance of the spores to lytic enzymes, to ether, and to elevated temperature. Expression of the DIT and DIT2 genes is restricted to sporulating cells, with the DIT1 transcripts accumulating at the time of prospore enclosure and just prior to the time of dityrosine biosynthesis. Both genes act in a spore-autonomous manner implying that at least some of the activities responsible for forming the outermost layer of the spore wall reside within the developing spore rather than in the surrounding ascal cytoplasm. As the DIT2 gene product has significant homology with cytochrome P-450s, DIT2 may be responsible for catalyzing the oxidation of tyrosine residues in the formation of dityrosine.

Control of cell growth and division in Saccharomyces cerevisiae

Considerable advances have been made in recent years in our understanding of the biochemistry of protein and nucleic acid synthesis and, particularly, the molecular biology of gene expression in eukaryotes. The yeast Saccharomyces cerevisiae, and to a lesser extent Schizosaccharomyces pombe, has had a preeminent role as a focus for these studies, principally because of the facility with which these organisms can be experimentally manipulated biochemically and genetically. This review will be designed to critically examine and integrate recent advances in several vital areas of regulatory control of enzyme synthesis in yeast: structure and organization of DNA, transcriptional regulation, post-transcriptional modification, control of translation, post-translational modification and secretion, and cell-cycle modulation. It will attempt to emphasize and illustrate, where detailed information is available, principal underlying molecular mechanisms, and it will attempt to make relevant comparisons of this material to inferred and demonstrated facets of regulatory control of enzyme and protein synthesis in higher eukaryotes.