- Volume 137, Issue 2, 1991

Addition of glucose or related fermentable sugars to derepressed cells of the yeast Saccharomyces cerevisiae triggers a RAS-protein-mediated cAMP signal, which induces a protein phosphorylation cascade. Yeast strains without a functional CDC25 gene were deficient in basal cAMP synthesis and in the glucose-induced cAMP signal. Addition of dinitrophenol, which in wild-type strains strongly stimulates in vivo cAMP synthesis by lowering intracellular pH, did not enhance the cAMP level. cdc25 disruption mutants, in which the basal cAMP level was restored by the RAS2 va119 oncogene or by disruption of the gene (PDE2) coding for the high-affinity phosphodiesterase, were still deficient in the glucose- and acidification-induced cAMP responses. These results indicate that the CDC25 gene product is required not only for basal cAMP synthesis in yeast but also for specific activation of cAMP synthesis by the signal transmission pathway leading from glucose to adenyl cyclase. They also show that intracellular acidification stimulates the pathway at or upstream of the CDC25 protein. When shifted to the restrictive temperature, cells with the temperature sensitive cdc25-5 mutation lost their cAMP content within a few minutes. After prolonged incubation at the restrictive temperature, cells with this mutation, and also those with the temperature sensitive cdc25-1 mutation, arrested at the ‘start’ point (in G1) of the cell cycle, and subsequently accumulated in the resting state G0. In contrast with cdc25-5 cells, however, the cAMP level did not decrease and normal glucose- and acidification-induced cAMP responses were observed when cdc25-1 cells were shifted to the restrictive temperature. These results show that, in the original genetic background at least, growth arrest of cdc25-1-bearing cells at the restrictive temperature is not due to cAMP deficiency. Previous experiments have provided evidence for the presence of a glucose-repressible protein in the signalling pathway. Exponential-phase glucose-grown cells of a strain with overexpression of CDC25 unexpectedly showed a glucose-induced cAMP signal. Control experiments, however, indicated that overexpression of CDC25 caused a defect in glucose repression. Introduction of the cat1 derepression mutation in the strain with overexpression of CDC25 restored glucose repression and abolished the glucose-induced cAMP signal, indicating that the CDC25 protein is not the glucose-repressible component of the signalling pathway.

Myxococcus xanthus, when starved on a solid surface, undergoes a multicellular cycle of development that consists of fruiting and sporulation. Myxospore germination has been followed in CTT, a complex medium composed mainly of peptides, by monitoring the sequential disappearance of some characteristic spore properties. Loss of heat resistance, shortly after incubation was initiated, was followed by loss of resistance to SDS and finally, refractility and ovoid shape. Germination of a population of myxospores did not occur synchronously. However, in the presence of calcium, germination was activated, being more rapid and synchronous. Other spore activation treatments tested did not have the same stimulatory effect. We searched for specific nutrients or chemicals capable of inducing germination. Amino acids, unlike other potential carbon and energy sources for M. xanthus, or several sugars tested, were most effective in triggering germination. Continuous incubation in CTT and Casamino acids germinant solutions was not required for induction and completion of germination of a large proportion of spores in a population.

Growth of Synechocystis PCC 6803 in BG-11 medium supplemented with 1 mM-CsCl resulted in intracellular accumulation of Cs+ to a final level of approximately 510 nmol (109 cells)-1 after incubation for 10 d. The doubling time was increased by 64% and the final cell yield was decreased by 70% during growth in the presence of Cs+ as compared to growth in control BG-11 medium. When the total monovalent cation concentration of the medium was doubled by adding either K+ or Na+, levels of accumulated Cs+ were decreased by approximately 50% to 220 and 270 nmol (109 cells)-1, respectively, after 28 d with little inhibition of growth being apparent. Short-term experiments revealed that extracellular K+ and Na+ inhibited Cs+ accumulation to a similar extent, with 90% inhibition of Cs+ accumulation occurring at the highest concentrations used (50 mM-K+ or Na+; 1 mM-Cs+). In all experiments, Cs+ accumulation resulted in a reduction in intracellular K+, except when cells were grown in K+-depleted medium, although a stoichiometric relationship was not apparent, the amount of Cs+ accumulated generally being greater than the amount of K+ released. Cs+ accumulation had no discernible effect on intracellular Na+. When K+, Na+, Rb+, Li+ or Tl+ were supplied at equimolar (1 mM) concentrations to Cs+, only Tl+ significantly reduced Cs+ accumulation. However, an approximately 50% inhibition of Cs+ accumulation resulted when concentrations of K+, Na+, Rb+ or Li+ were increased to 10 mM, which suggests that Cs+ may have a higher affinity for the monovalent cation transport system than K+, Rb+ and TI+ also caused a decrease in intracellular K+, whereas Na+ and Li+ stimulated K+ uptake. Cs+ accumulation was dependent on the external Cs+ concentration and showed a linear relationship to external Cs+ concentrations≤2 mM over 12 h incubation. However, prolonged incubation in external Cs+ concentrations≥ 0·8 mM resulted in Cs+ release from the cells and after 48 h, similar amounts of Cs+ and K+ were present in cells incubated at these higher concentrations. Cs+ accumulation was energy- and pH-dependent. Incubation in the light at 4 °C, or in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), or at 22 °C in the dark resulted in decreased Cs+ accumulation and decreased K+ release from the cells. Increased amounts of Cs+ were accumulated as the pH of the external medium was increased, with maximal accumulation [approximately 1330 nmol Cs+ (109 cells)-1 after 24 h incubation] occurring at pH 10, the highest pH value used. It is suggested that an important mechanism of Cs+ toxicity in Synechocystis PCC 6803 arises through replacement of cellular K+ by Cs+. The possible role of primary producers such as cyanobacteria in the mobilization of this radionuclide in aquatic habitats is discussed.