EFFECTS OF TEMPERATURE SHIFTS ON THE ACTIVITIES OF NEUROSPORA-CRASSA GLYCOGEN-SYNTHASE, GLYCOGEN-PHOSPHORYLASE AND TREHALOSE-6-PHOSPHATE SYNTHASE

Conidiospore germlings of Neurospora crassa submitted to a heat shock at 45 degrees C accumulate trehalose and degrade glycogen. The opposit e occurs upon reincubation at a physiologic temperature (30 degrees C) . These observations suggest a temperature-dependent mechanism for the preferential synthesis of one or the other sugar reserve. Here we sho w that concomitant with these shifts of temperature, occurred reversib le changes in the activities of glycogen synthase and phosphorylase. G lycogen synthase was inactivated at 45 degrees C while phosphorylase w as activated. The reverse was true when the cells were shifted back to 30 degrees C. Addition of cycloheximide did not prevent the reversibl e enzymatic changes, which remained stable after gel filtration. Appar ently, the effects of temperature shifts occurred at the level of reve rsible covalent enzymatic modifications. Trehalose-6-phosphate synthas e properties were also affected by temperature. For instance, the enzy me was less sensitive to in vitro inhibition by inorganic phosphate at 50 degrees C than at 30 degrees C. Fructose-6-phosphate partially rel ieved the inhibitory effect of phosphate at 30 degrees C but not at 50 degrees C. These effects of the assay temperature, inorganic phosphat e, and fructose-6-phosphate, on trehalose-6-phosphate synthase activit y, were more evident for crude extracts obtained from heat-shocked cel ls. Altogether, these results may contribute to explain the preferenti al accumulation of trehalose 15 degrees C, or that of glycogen at 30 d egrees C.