Transcription factors (TFs) are among the most frequently detected targets of sumoylation, and effects of the modification have been studied for about 200 individual TFs to date. TF sumoylation is most often associated with reduced target gene expression, which can be mediated by enhanced interactions with corepressors or by interference with protein modifications that promote transcription. However, recent studies show that sumoylation also regulates gene expression by controlling the levels of TFs that are associated with chromatin. SUMO can mediate this by modulating TF DNA-binding activity, promoting clearance of TFs from chromatin, or indirectly, by influencing TF abundance or localization.

Numerous proteins are sumoylated in normally growing yeast and SUMO conjugation levels rise upon exposure to several stress conditions. We observe high levels of sumoylation also during early exponential growth and when nutrient-rich medium is used. However, we find that reduced sumoylation (∼75% less than normal) is remarkably well-tolerated, with no apparent growth defects under nonstress conditions or under osmotic, oxidative, or ethanol stresses. In contrast, strains with reduced activity of Ubc9, the sole SUMO conjugase, are temperature-sensitive, implicating sumoylation in the heat stress response, specifically. Aligned with this, a mild heat shock triggers increased sumoylation which requires functional levels of Ubc9, but likely also depends on decreased desumoylation, since heat shock reduces protein levels of Ulp1, the major SUMO protease. Furthermore, we find that a ubc9 mutant strain with only ∼5% of normal sumoylation levels shows a modest growth defect, has abnormal genomic distribution of RNA polymerase II (RNAPII), and displays a greatly expanded redistribution of RNAPII after heat shock. Together, our data implies that SUMO conjugations are largely dispensable under normal conditions, but a threshold level of Ubc9 activity is needed to maintain transcriptional control and to modulate the redistribution of RNAPII and promote survival when temperatures rise.

The ammonolysis of several substituted phenyl thiolacetates is kinetically studied in aqueous medium, 18 • C, ionic strength 0.1 M (KCl). By following the leaving groups spectrophotometrically (λ max = 260-410 nm), under excess free ammonia, pseudo-first-order rate coefficients (k obs ) are obtained. The plots of (k obs − k H ) against free ammonia concentration are linear at constant pH. The macroscopic nucleophilic substitution rate coefficients (k N ) are obtained as the slopes of these plots and found to be pH-independent for all the thiolesters. The Brönsted-type plot (log k N against pK a of leaving groups) and the Hammett plot (log k N against σ values of substituents) obtained for the title reactions of thiolesters are linear with slope values of β lg = −0.34 and ρ = 0.74 respectively. From the magnitude of these values, experimental data, the kinetic law, and the analysis of products, it is deduced that the ammonolysis of thiolesters proceeds through a simple bimolecular nucleophilic substitution pathway with a zwitterionic tetrahedral addition intermediate (T ± ), whereby its formation is rate-determining (k 1 step). Comparison of this reaction of thiolesters with a similar reaction of analogue oxyesters shows a mechanistic difference. Further, for thiolesters there is a rate enhancement with larger k N values. The change in mechanism and enhanced reactivity observed by substitution of the oxygen atom by sulphur atom on the phenyl moiety are discussed in detail.

Sumoylation is an essential, conserved protein modification with hundreds of targets. Compared to the related modification ubiquitination, few enzymes are involved in SUMO conjugation and deconjugation, including a single conjugase, Ubc9, in yeast and mammals. This suggests that cells can simultaneously control the sumoylation level of numerous proteins by regulating just one enzyme of the SUMO pathway. Such modulated levels of cellular sumoylation are observed in response to a number of stress conditions, which typically cause a rapid and dramatic increase in overall sumoylation. Here, we demonstrate that ploidy, culture density, and nutrient availability also affect global sumoylation levels in yeast. To determine the effects of modulated cellular sumoylation on cell growth, we examined yeast strains that artificially display elevated or reduced sumoylation. Remarkably, reducing sumoylation levels by >70% has no effect on cell fitness, indicating that most SUMO conjugation events are not required for normal growth.Surprisingly, among a panel of stress conditions, only elevated temperatures severely impacted growth of cells harbouring constitutively low sumoylation levels. Consistent with the fact that Ubc9 and SUMO are essential, however, cells displaying less than ~5% of normal sumoylation levels show significantly impaired growth, even at normal temperatures. Finally, we demonstrate that many sumoylation events are highly transient, requiring constant de novo sumoylation to maintain steady state levels. Together, our results suggest that cells need only a low basal level of sumoylation for growth, but that normal levels are required pre-emptively to facilitate survival at elevated temperatures.

The oscillatory phenomenon during oxidation of gallic acid with potassium bromate, catalyzed by cerium ion in acid medium with stirring was investigated. The cross section of a three dimensional graph for the regions where the oscillatory behavior can be observed is given. Effect of Ag+and Br− on the oscillatory behavior was studied.