A Schizosaccharomyces pombe gene, ksg1, that shows structural homology to the human phosphoinositide-dependent protein kinase PDK1, is essential for growth, mating and sporulation

Fission yeast (Schizosaccharomyces pombe) requires inositol for growth, mat ing and sporulation. To define putative genes that are involved in the proc essing and transduction of the inositol signal, mutants that are temperatur e sensitive for growth and sporulation were selected on a medium containing non-limiting amounts of inositol. Two such mutants (ksg1-208 and ksg1-358) were analyzed, which are impaired in mating and sporulation at 30 degrees C and undergo growth arrest in the G2 phase of the cell cycle at 35 degrees C. The ksg1 gene was isolated by functional complementation. It maps on th e left arm of chromosome II and encodes a putative 592-amino acid protein w hich exhibits good structural homology to a human 3-phosphoinositide-depend ent protein kinase (PDK1) and its rat and Drosophila homologues. The two mu tants have the same substitution at amino acid position 159: a glycine resi due is replaced by glutamic acid. Deletion of the gene is lethal for haploi d cells. We propose that ksg1 is involved in one or several phosphoinositid e signalling processes that are responsible for control of the life cycle.