EXOGENOUS INOSITOL AND GENES RESPONSIBLE FOR INOSITOL TRANSPORT ARE REQUIRED FOR MATING AND SPORULATION IN SCHIZOSACCHAROMYCES-POMBE

Fission yeast, Schizosaccharomyces pombe, is a natural inositol auxotr oph. We show here that the amount of exogenous inositol added to the m edium is critical for the control of its life cycle. Above growth-limi ting concentrations inositol stimulates mating and sporulation in mini mal medium. The effect of inositol is also observed on yeast-extract-m edium plates. We selected a mutant, IM49, which mates and sporulates o nly poorly and show that it is defective in inositol transport. Its de fect is in a gene (itr2) coding for a putative 12 membrane-spanning pr otein. The polypeptide contains the two sugar-transport motifs typical for hexose transporters and shows good homology to the two Saccharomy ces cerevisiae inositol transporters. The itr2 gene is essential for c ell growth and its mRNA level is repressed by glucose. Mutant IM49 is also complemented by a multicopy suppressor gene (itr1) which codes fo r a putative hexose transporter with unknown substrate specifity.