Restoration of inositol prototrophy in the fission yeast Schizosaccharomyces pombe.

The biosynthesis of inositol requires only two enzymes, inositol-1-phosphate synthase (encoded by INO1) and an inositol monophosphatase, but the regulation of inositol biosynthesis is under multiple controls and is exquisitely regulated. In the budding yeast Saccharomyces cerevisiae, mutations in any of 26 different genes lead to inositol auxotrophy. The fission yeast Schizosaccharomyces pombe, however, is a natural inositol auxotroph. An investigation has been initiated to examine the possible reasons that might have led to inositol auxotrophy in Sch. pombe. Complementation with a genomic library of an inositol prototrophic yeast indicated that a Pichia pastoris INO1 gene alone could confer inositol prototrophy to Sch. pombe and that the gene was absent in Sch. pombe. To investigate possible reasons for the loss of INO1 gene in Sch. pombe, an attempt was made to disrupt inositol homeostasis in Sch. pombe by overproduction of intracellular inositol, but this did not lead to any discernible adverse effects. The sources of inositol in the natural environment of Sch. pombe were also examined. As the natural environment of Sch. pombe contains significant amounts of phytic acid (inositol hexaphosphate), an investigation was carried out and it was discovered that Sch. pombe can utilize phytic acid as a source of inositol under very specific conditions.