Cyclic AMP, fructose-2,6-bisphosphate and catabolite inactivation of enzymes in the hydrocarbon-assimilating yeast Candida maltosa

Expression profiling reveals Spot 42 small RNA as a key regulator in the central metabolism of Aliivibrio salmonicida

BACKGROUND

Spot 42 was discovered in Escherichia coli nearly 40 years ago as an abundant, small and unstable RNA. Its biological role has remained obscure until recently, and is today implicated in having broader roles in the central and secondary metabolism. Spot 42 is encoded by the spf gene. The gene is ubiquitous in the Vibrionaceae family of gamma-proteobacteria. One member of this family, Aliivibrio salmonicida, causes cold-water vibriosis in farmed Atlantic salmon. Its genome encodes Spot 42 with 84% identity to E. coli Spot 42.

RESULTS

We generated a A. salmonicida spf deletion mutant. We then used microarray and Northern blot analyses to monitor global effects on the transcriptome in order to provide insights into the biological roles of Spot 42 in this bacterium. In the presence of glucose, we found a surprisingly large number of ≥ 2X differentially expressed genes, and several major cellular processes were affected. A gene encoding a pirin-like protein showed an on/off expression pattern in the presence/absence of Spot 42, which suggests that Spot 42 plays a key regulatory role in the central metabolism by regulating the switch between fermentation and respiration. Interestingly, we discovered an sRNA named VSsrna24, which is encoded immediately downstream of spf. This new sRNA has an expression pattern opposite to that of Spot 42, and its expression is repressed by glucose.

CONCLUSIONS

We hypothesize that Spot 42 plays a key role in the central metabolism, in part by regulating the pyruvat dehydrogenase enzyme complex via pirin.

Glucose-induced inactivation of isocitrate lyase in Aspergillus nidulans

The existence of a second mechanism of catabolite control of isocitrate lyase of Aspergillus nidulans, in addition to the carbon catabolite repression phenomenon recently reported was analysed. Isocitrate lyase was rapidly and specifically inactivated by glucose. The inactivation was irreversible at all stages in the presence of cycloheximide, showing that reactivation depends on de novo protein synthesis. In addition, analysis of glucose-induced inactivation of isocitrate lyase in a creAd-30 strain showed that the creA gene is not involved in this process.

Catabolite inactivation, cyclic AMP and protein phosphorylation in the methylotrophic yeast Hansenula polymorpha

The inactivation of the peroxisomal enzyme alcohol oxidase and the cytoplasmic enzymes fructose-1,6-bisphosphatase, malate dehydrogenase and phosphoenolpyruvate carboxykinase was found to occur after addition of glucose to methanol-grown cells of the yeast Hansenula polymorpha. The concentration of cyclic AMP increased nearly twofold within 3 min under the same conditions. In crude extracts of H. polymorpha about 20 proteins are phosphorylated by cyclic AMP dependent protein kinases, among them also fructose-1,6-bisphosphatase. No phosphorylation of the alcohol oxidase protein could be detected. From this fact, it was concluded that the inactivation of the peroxisomal alcohol oxidase is independent of cyclic AMP-dependent protein phosphorylation.

Catabolite Inactivation in the Methylotrophic Yeast Pichia pastoris

Inactivation of the alcohol oxidase enzyme system of Pichia pastoris , during the whole-cell bioconversion of ethanol to acetaldehyde, was due to catabolite inactivation. Electron microscopy showed that methanol-grown cells contained peroxisomes but were devoid of these microbodies after the bioconversion. Acetaldehyde in the presence of O 2 was the effector of catabolite inactivation. The process was initiated by the appearance of free acetaldehyde, and was characterized by an increase in the level of cyclic AMP, that coincided with a rapid 55% drop in alcohol oxidase activity. Further enzyme inactivation, believed to be due to proteolytic degradation, then proceeded at a constant but slower rate and was complete 21 h after acetaldehyde appearance. The rate of catabolite inactivation was dependent on acetaldehyde concentration up to 0.14 mM. It was temperature dependent and occurred within 24 h at 37°C and by 6 days at 15°C but not at 3°C. Alcohol oxidase activity was psychrotolerant, with only a 17% decrease in initial specific activity over a temperature drop from 37 to 3°C. In contrast, protease activity was inhibited at temperatures below 15°C. When the bioconversion was run at 3°C, catabolite inactivation was prevented. In the presence of 3 M Tris hydrochloride buffer, 123 g of acetaldehyde per liter was produced at 3°C, compared with 58 g/liter at 30°C. By using 0.5 M Tris in a cyclic-batch procedure, 140.6 g of acetaldehyde was produced.

[Activities of gluconeogenetic enzymes in the yeast Candida maltosa during growth on glucose or ethanol]

The activities of fructose-1,6-bisphosphatase, malate dehydrogenase and PEP carboxykinase were tested during discontinuous growth of the n-alkane-assimilating yeast Candida maltosa on glucose or ethanol. As expected, the highest activities of the enzymes were measured in the early log phase of growth on ethanol and the lowest in the early log phase of growth on glucose. However, the differences in the activities are much smaller than in Saccharomyces cerevisiae and other yeasts under similar conditions. Therefore, we conclude that catabolite repression does not play an essential role in the control of gluconeogenesis in Candida maltosa.

Cyclic AMP-dependent phosphorylation of fructose-1,6-bisphosphatase and other proteins in the yeast Candida maltosa

In crude extracts of Candida maltosa, about 12 proteins are phosphorylated in the presence of cAMP or of a catalytic subunit of cAMP-dependent protein kinase. A strongly labelled protein spot occurred in the position of fructose-1,6-bisphosphatase both after electrophoresis of crude extracts incubated with cAMP and of a partially purified fructose-1,6-bisphosphatase incubated with a catalytic subunit of cAMP-dependent protein kinase. No phosphorylation of the cytoplasmic malate dehydrogenase could be detected. From these results it was concluded that cAMP-dependent phosphorylation plays an important role in the catabolite inactivation of fructose-1,6-bisphosphatase in Candida maltosa, as described for Saccharomyces cerevisiae.