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/MATalpha) but does not appear in vegetatively growing cells or in nonsporulating diploids (MATalpha/MATalpha) 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.