Addition of glucose or other substrates to starved Saccharomyces cerevisiae cells triggers a cyclic AMP signal which induces the protein phosphorylating cascade. Before the addition of various substrates the wild-type and mutant yeast strains were arrested at the G1 phase of the cell division cycle by transferring the cells, grown at 26 degrees C to 36 degrees C in a synthetic medium without any substrate. After the temperature shift back to 26 degrees C different substrates were added and the cAMP levels were measured. The highest cAMP levels were observed immediately after the addition of the substrates. A relationship between the maximum growth rate of the individual strains or mutants at a given substrate and the intracellular cAMP level is discussed.

Protein kinases represent a diverse family of enzymes that play a critical role in regulation. Among nearly 100 known protein kinases, the cAMP-dependent enzyme is best understood biochemically. Unlike other protein kinases, cAMP-dependent protein kinase consists of two different types of subunits that dissociate, a regulatory subunit (R), which is the receptor for cAMP, and a catalytic subunit (C). In the absence of cAMP, the enzyme exists as an inactive tetramer, R2C2. The binding of intracellular cAMP to the R subunit decreases the affinity of the R subunit for the C subunit by approximately four orders of magnitude and, under physiological conditions, leads to dissociation of the holoenzyme into R2(cAMP)4 dimer and two free C subunits that are catalytically active. Mutants of the cAMP metabolism, adenylate cyclase and cell cycle mutants, provided further information about protein synthesis and cellular growth in Saccharomyces cerevisiae. The purified protein kinases were divided into different types according to their elution profiles from the DEAE-cellulose matrix. Two types of cAMP-dependent and two types of cAMP-independent protein kinases were isolated from the wild strain. Differences in the activities of the kinases in the mutants showed a close relationship to the locus of the respective mutations in the cell-cycle. Some properties of the protein kinases are discussed with respect to individual mutations.

The addition of D(+)-glucose (final concentration 50 mM) to a cell suspension of yeasts (wild type and several mutants of the cell cycle, the cAMP-dependent protein kinase system, and a mutant of the adenylate cyclase gene) triggers a rapid increase in the concentrations of cAMP and cGMP in the wild strain. In contrast to cAMP, an increase of cGMP was also found in the mutants. cAMP and cGMP have been characterized as second messengers in eucaryotic cells. Cyclic nucleotide activation of the protein kinases enables them to perform their only known function in eukaryotes, the phosphorylation of substrate proteins. The results, described here by using selected yeast mutants as a model for higher eukaryotes, indicate that there exist two different regulatory systems for the control of the cAMP and cGMP levels.