SSG1, A GENE ENCODING A SPORULATION-SPECIFIC 1,3-BETA-GLUCANASE IN SACCHAROMYCES-CEREVISIAE

In Saccharomyces cerevisiae, the meiotic process is accompanied by a l arge increase in 1,3-beta-glucan-degradative activity. The molecular c loning of the gene (SSG1) encoding a sporulation-specific exo-1,3-beta -glucanase was achieved by screening a genomic library with a DNA prob e obtained by polymerase chain reaction amplification using synthetic oligonucleotides designed according to the nucleotide sequence predict ed from the amino-terminal region of the purified protein. DNA sequenc ing indicates that the SSG1 gene specifies a 445-amino-acid polypeptid e (calculated molecular mass, 51.8 kDa) showing extensive similarity t o 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. Ne breda, E. Mendez, and F. del Rey, Gene 97:173-182, 1991). The N-termin al domain of the putative precursor is a very hydrophobic segment with structural features resembling those of signal peptides of secreted p roteins. Northern (RNA) analysis reveals a unique SSG1-specific transc ript, 1.7 kb long, which can be detected only in sporulating diploids (MATa/MATalpha) but does not appear in vegetatively growing cells or i n nonsporulating diploids (MATalpha/MATalpha) when incubated under nit rogen 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 du ring spore formation. Homozygous ssg1/ssg1 mutant diploids are able to complete sporulation, although with a significant delay in the appear ance of mature asci.