Microbiology Monthly Newsletter for Microbiology at the Department of Cell & Molecular Biology Lundberg Laboratory, Göteborg University [ January 1999 ] [ February 1999 ] [ March 1999 ] [ April 1999 ] [ May 1999 ] [ June 1999 ] [ July-August 1999 ] [ September 1999 ] [ October 1999 ] April 1999 Scientific Contributions This month there is a number of new papers on our paper display from the yeast syndicate. In one paper, Stefan Hohmann and colleagues from Belgium and South Africa elucidate the signalling pathways responsible for the regulation of GPDI expression during changes in medium osmolarity. GPD1, as some of you may have noticed, encodes a glycerol-3-phosphate dehydrogenase which is required for the accumulation of glycerol during osmotic shock of yeast cells. It is demonstrated that the HOG (High Osmolarity Glycerol) pathway is a key player in GPD1 regulation but that other and hitherto unknown regulatory pathways have additional roles. For example, GPD1 is till induced by an osmotic shock in HOG1 deletion mutants albeit with delayed kinetics and a lower magnitude of induction. Also, reduced transcription of GPD1 during hypo-osmotic treatments is independent of HOG1. A number of other regulators with known roles in osmoregulation were found to have no or only marginal roles in GPD1 regulation. Thus, there will be more to learn about the regulation of GPD1 and glycerol accumulation in osmotically stressed yeast cells. By the way, GPDH, as the glycerol-3-phosphate dehydrogenase gene is called in many organisms, appears to be a very slowly evolving gene, at least in Drosophila species. It has been estimated that the rate of evolution of GPDH is about 10 amino acid replacements per 1200 millions of years. In comparison SOD, encoding a superoxide dismutase, have had the same number of replacements in only 50 million years, approximately. Interesting, huh? In another paper, also linked to the problem of regulating cellular water content, Vincent, Stefan and co-workers from France, present a molecular and functional study of a yeast gene called AQY1. From data of the genome project of Saccharomyces cerevisiae it was deduced that the product of AQY1 may be a protein of the MIP (Major Intrinsic Protein) family. In addition, the protein encoded by the gene, Aqy1p, is highly homologues to membrane aquaporin proteins of Arabidopsis thaliana. The AQY1 gene was cloned and functional analysis indicated that this gene, indeed, encodes a yeast aquaporin. The paper also reports on a significant aquaporin gene heterogeneity among different yeast strains and that there may be a second functional aquaporin in some strains. More to come, I’m sure. Who would have guessed that the Rainbow Trout intestine harbors cells of Saccharomyces cerevisiae. Well, it does, and Thomas Andlid, Lena Blomberg, Lena Gustafsson, and Anders Blomberg characterize one such yeast isolated from the fish. The growth characteristics of this strain in comparison to a well-defined laboratory strain were found to be similar. However, as judged by microcalorimetric analysis, the ”fish” strain reduces its metabolic activity earlier in the respiratory phase. In addition, 2D-PAGE analysis showed that the levels of protein production was slightly different. Notably, the ”fish” isolate exhibited an enhanced production of heat shock proteins. This isolate also made 12 proteins that were apparently absent in the laboratory strain in the growth conditions analyzed. Finally, we enter the somewhat elusive subject of catabolite repression in yeast. Stefan and co-workers report that missense mutations in HXK2, encoding hexokinase PII, suppress the inability of tps1D hxk1D double mutants to grow on fructose. The HXK2 gene has previously been demonstrated to be required for glucose catabolite repression in yeast. However, the analysis of the new hxk2 mutant’s ability to confer catabolite repression indicates that the role of the hexokinase is most likely not exclusively linked to the production (or concentrations) of sugar phosphates. In addition, the paper reports on the lack of correlation between cAMP signal and catabolite repression. It is interesting to compare this with E. coli which appear to ”sense” the PEP/pyruvate ratio dictating the degree of phosphorylation of protein IIA. When phopshorylated, this protein activates adenylate cyclase, cAMP accumulates and catabolite repressible genes are derepressed (activated really). Thus, E. coli has it backwords (or yeast if you like). Happy Valborg - Don’t overdo it!