Genetic relationship between the SPT3 gene and RAS/cAMP pathway in yeast cell cycle control

The signal transduction pathways through the RAS gene product and adenyl cyclase play a critical role in regulation of the cell cycle in yeast, Saccharomyces cerevisiae. We examined the genetic relationship between the spt3 gene and ras/cAMP pathway. A mutation in the SPT3 gene suppressed cell cycle arrest at the G1 phase caused by either an inactivation of the RAS or CYR1 gene which encodes a yeast homologue of human ras proto-oncogene or adenyl cyclase, respectively. The phenotypes such as sporulation and heat shock resistancy, that resulted from a partial inactivation of the RAS or CYR1 genes, were also suppressed by the spt3 mutation. Expression of the SSA1 gene encoding one of the heat shock proteins (Hsp70) can be induced by heat shock or nitrogen starvation. Expression of this gene is derepressed in cyr1-2 and spt3 mutants. The bcy1 mutation repressed heat inducibility of SSA1 expression. The high basal expression of SSA1 in cyr1-2 was suppressed by the bcy1 mutation, but not in spt3 mutants. These results suggest that the SPT gene is involved in expression of genes that are affected by the RAS/cAMP pathway.ope