In this study, the effects of inositol addition on invertase secretion, biosynthesis of mRNA encoding secreted invertase and phosphatidylinositol (PI) biosynthesis in Schizosaccharomyces pombe were examined. We found that specific invertase activity was at its maximum when the concentration of added inositol reached 800 mug/100 ml in the synthetic medium containing 2.0% (w/v) sucrose. However, cell growth was steadily increased as inositol concentrations were increased from 0 to 1000 mug/100 ml. When the concentration of added inositol was 100 mug/100 ml in the synthetic medium, repression of invertase secretion occurred at sucrose concentration higher than 0.5% (w/v). However. when S. pombe was cultured in the synthetic medium containing 800 mug/100 ml, repression of invertase secretion occurred only at sucrose concentrations above 2.0% (w/v). Less residual reducing sugar was left in the 800 mug/100 ml inositol-containing medium than in the 100 mug/100 ml inositol-containing medium as sucrose concentration was increased. More mRNA encoding secreted invertase was detected in cells grown in the 800 mug/100 ml inositol-containing medium (2.0% sucrose) than in those grown in the 100 mug/100 ml inositol-containing medium. These results demonstrate that higher concentration of added inositol in the synthetic medium could derepress invertase secretion and INV+ gene expression in S. pombe. PI content of the yeast cells grown in the synthetic medium with 800 mug/ 100 ml of inositol was higher than that of the yeast cells grown in the same medium with 100 mug/100 ml of inositol. This means that PI may be involved in derepression of invertase secretion. (C) 2003 Elsevier Inc. All rights reserved.