REGULATION OF CHITIN SYNTHASE ACTIVITY IN THE DIMORPHIC FUNGUS BENJAMINIELLA-POITRASII BY EXTERNAL OSMOTIC-PRESSURE

The effects of changes in external osmotic pressure on chitin synthase activity of a dimorphic fungus, Benjaminiella poitrasii, have been in vestigated. Mycelial and yeast cells incubated in medium oi. low osmol ality (distilled water, 0 mOsm) for 10 min had 2-3-fold higher specifi c activities of native chitin synthase in mixed membrane preparations than cells that had been subjected to a high osmolality medium (1.2 M sorbitol in distilled water, 1612 mOsm). Cells suspended in media of d ifferent osmolalities for 10 min were also affected in the extent of g erm tube formation. Germ tube formation was highest in cells incubated in low osmolality medium. The addition of protein phosphatase inhibit ors (cyclosporin A, 1.2 mu g/ml; cantharidin, 20 mu M) abolished the e ffect of hypo-osmotic stress on chitin synthase activation of yeast mi xed membrane preparations. The presence of protein kinase inhibitors ( genistein, 40 mu g/ml; H-7, 100 mu M) and a Ca2+ channel blocker (vera pamil, 50 mu M) reduced chitin synthase activity to 50-60% of that obs erved in cells under hypo-osmotic shock. These inhibitors also inhibit ed germ tube formation. This suggests that chitin synthase activity an d yeast hyphal morphogenesis are both subject to regulation by osmotic pressure, phosphorylation and calcium.