Mechanism In Yeast Research Articles

Evolutionary divergence of the mRNA transcription initiation mechanism in yeast.

The promoters of eukaryotic genes are being increasingly defined through the identification of consensus DNA sequences, by mutational analysis, and by in vitro and in vivo studies of transcription. Whereas the TATA sequence (Goldberg-Hogness box) has been largely conserved among protein encoding genes (transcribed by RNA polymerase II) of eukaryotes, there is some evidence that other structural and functional determinants of mRNA transcription are not conserved between species. I report there an in vivo comparative analysis of the transcription initiation systems of the budding yeast Saccharomyces cerevisiae and the fission yeast Schizosaccharmyces pombe (which can both be transformed by identical plasmids). I have found no instance in which a gene is transcribed in the same fashion in both yeasts. Instead, I have found that the in vivo transcription starting points for many different yeast genes are determined by the cell in which it is transcribed rather than its gene structure alone. The evidence also suggests that the divergence of the transcription initiation system may partly involve the mechanism or structure which determines the distance from the TATA consensus sequence to the site of transcription initiation.

Physiological Effects of Seven Different Blocks in Glycolysis inSaccharomyces cerevisiae

Saccharomyces cerevisiae mutants unable to grow and ferment glucose have been isolated. Of 45 clones isolated, 25 had single enzyme defects of one of the following activities: phosphoglucose isomerase (pgi), phosphofructokinase (pfk), triosephosphate isomerase (tpi), phosphoglycerate kinase (pgk), phosphoglyceromutase (pgm), and pyruvate kinase (pyk). Phosphofructokinase activities in crude extracts of the pfk mutant were only 2% of the wild-type level. However, normal growth on glucose medium and normal fermentation of glucose suggested either that the mutant enzyme was considerably more active in vivo or, alternatively, that 2% residual activity was sufficient for normal glycolysis. All other mutants were moderately to strongly inhibited by glucose. Unusually high concentrations of glycolytic metabolites were observed before the reaction catalyzed by the enzyme which was absent in a given mutant strain when incubated on glucose. This confirmed at the cellular level the location of the defect as determined by enzyme assays. With adh (lacks all three alcohol dehydrogenase isozymes) and pgk mutants, accumulation of the typical levels of hexosephosphates was prevented when respiration was blocked with antimycin A. A typical feature of all glycolytic mutants described here was the rapid depletion of the intracellular adenosine 5'-triphosphate pool after transfer to glucose medium. No correlation of low or high levels of fructose-1,6-bisphosphate with the degree of catabolite repression and inactivation could be found. This observation does not support the concept that hexose metabolites are directly involved in these regulatory mechanisms in yeast.

Metabolic control mechanisms in yeast grown with different glucose concentrations

A strain of S. carlsbergensis grown with 0.6 or 2.0% glucose in the culture medium exhibited different control mechanisms during aerobic glycolysis, although the kinetics of oxygen consumption were qualitatively the same. However, the kinetics of the carriers of the respiratory chain (with equal cytochrome concentration in both yeasts) were found to be different. In the yeast grown with 0.6% glucose a reduction of the carriers during the Crabtree effect pointed to phosphate or phosphate acceptor control. In the yeast grown with 2.0% glucose an oxidation of the carriers pointed to control in the supply of substrates for the respiratory chain, i.e., control in the glycolytic pathway.

Photoreactivation studies on yeasts: Ultraviolet inactivation and photoreactivation of respiration-sufficient and respiration-deficient yeasts

This study compares the ultraviolet inactivation and photoreactivability of respiration-sufficient (AER) and respiration-deficient (cytochrome-deficient) (aer) yeasts. Attention is focused ( a) on the effect of different doses of photoreactivating light on the survival of irradiated cells of each type and ( b) on photoreactivation rate curves. AER cells were more resistant than the aer type to ultraviolet inactivation. The killing effect of ultraviolet on both AER and aer cells was partially reversed by photoreactivating light. The maximum per cent photoreactivation was approximately the same for both AER and aer cells and for young and stationary phase cultures. Thus a functional respiratory mechanism, and the cytochromes and respiratory enzymes lacking in the variant, are not obligatory components of the photoreactivation mechanism in yeast. The survival ratio of irradiated cells increased as an approximate exponential function of the dose of photoreactivating light until a plateau (maximum photoreactivation) was reached. With low doses of photoreactivating light, the survival ratio of aer cells increased at a rate twice that of AER cells. Photoreactivation rate curves also indicate that inactivated aer cells are reactivated more rapidly than inactivated AER cells. The decrease in frequency of aer variants induced by ultraviolet [13] cannot be accounted for by a difference in the photoreactivablity of the inactivation damage in AER and aer cells. Photoreactivation rate curves for haploid and tetraploid yeasts are multiple-hit in character, indicating that photoreactivation of the inactivation damage involves a complex mechanism.