NUTRIENT-INDUCED ACTIVATION OF TREHALASE IN NUTRIENT-STARVED CELLS OF THE YEAST SACCHAROMYCES-CEREVISIAE - CAMP IS NOT INVOLVED AS 2ND MESSENGER

Starvation of Saccharomyces cerevisiae cells for specific nutrients such as nitrogen, phosphate or sulphate causes arrest in the G1 phase of the cell cycle at a specific point called 'start'. Re-addition of different nitrogen sources, phosphate or sulphate to such starved cells causes activation of trehalase within a few minutes. Nitrogen-source- and sulphate-induced activation of trehalase were not associated with any change in the cAMP level, but in the case of phosphate there was a small transient increase. When nitrogen-source-activated trehalase was isolated by immuno-affinity chromatography from crude extracts, the purified enzyme showed the same activity profile as in the original crude extracts, indicating that post-translational modification is responsible for the activation. In the yeast mutants cdc25-5 and cdc35-10, which are temperature sensitive for cAMP synthesis, incubation at the restrictive temperature lowered but did not prevent nitrogen-, phosphate- or sulphate-induced activation of trehalase. Since under these conditions the cAMP level in the cells is very low, it is unlikely that cAMP acts as a second messenger in this nutrient-induced effect. Nitrogen-source-induced activation of trehalase requires the presence of glucose at a concentration similar to that able to stimulate the RAS-adenylate cyclase pathway. This indicates that the same glucose-sensing system might be involved in both phenomena. Nitrogen-starved cells fractionated according to cell size all showed nitrogen-source-induced activation of trehalase to the same extent, indicating that the nitrogen-induced signalling pathway involved is not dependent on the well-known cell size requirement for progression over the start point of the cell cycle.