THE LAG PHASE RATHER THAN THE EXPONENTIAL-GROWTH PHASE ON GLUCOSE IS ASSOCIATED WITH A HIGHER CAMP LEVEL IN WILD-TYPE AND CAPK-ATTENUATED STRAINS OF THE YEAST SACCHAROMYCES-CEREVISIAE

In the yeast Saccharomyces cerevisiae several phenotypic properties co ntrolled by cAMP-dependent protein kinase (cAPK) are indicative of hig h cAPK activity during growth on glucose and low activity during growt h on non-fermentable carbon sources and in stationary phase. It has be en a matter of debate whether the apparently higher activity of cAPK i n cells growing on glucose is due to a higher cAMP level or to an alte rnative mechanism activating cAPK. The cAMP level during diauxic growt h of yeast cells in cultures with different initial glucose levels and different initial cell densities has been reinvestigated and the prev iously reported twofold increase in cAMP during growth initiation has been confirmed, However, this increase was transient and entirely asso ciated with the lag phase of growth. The initiation of exponential gro wth on glucose was associated with a decrease in the cAMP level and th ere was no correlation between this decrease in cAMP and the depletion of glucose in the medium. In mutants defective in feedback inhibition of cAMP synthesis, resuspension of exponential-phase glucose-grown ce lls in glucose medium caused an extended lag phase during which a huge , transient accumulation of cAMP occurred, The latter required the pre sence of glucose and nitrogen, but not phosphate or sulfate, and was n ot due to intracellular acidification, as shown by in vivo P-31-NMR sp ectroscopy. The initiation of exponential growth on glucose was also a ssociated in this case with a decrease in cAMP rather than an increase . This behaviour was also observed in strains with attenuated catalyti c subunit activity and lacking the regulatory subunit and even in stra ins without catalytic subunits of cAPK. This might indicate that other mechanisms are able to cause down-regulation of cAMP synthesis in a w ay mimicking feedback inhibition. Transfer of glucose-growing cells of wild-type or cAPK-attenuated strains to a nitrogen starvation medium resulted in an increase in the cAMP level rather than a decrease. The results indicate that the apparent changes in cAPK activity in vivo du ring diauxic growth on glucose and during nitrogen starvation cannot b e explained on the basis of changes in the cAMP level.