Biased amino acid composition in warm-blooded animals

Among eubacteria and archeabacteria, amino acid composition is correlated with habitat temperatures. In particular, species living at high temperatures have proteins enriched in the amino acids E-R-K and depleted in D-N-Q-T-S-H-A. Here, we show that this bias is a proteome-wide effect in prokaryotes, and that the same trend is observed in fully sequenced mammals and chicken compared to cold-blooded vertebrates (Reptilia, Amphibia and fish). Thus, warm-blooded vertebrates likely experienced genome-wide weak positive selection on amino acid composition to increase protein thermostability

Liquid phase hydrogenation of crotonaldehyde over Pt/SiO2 catalysts

The dependence of the catalytic properties of Pt/SiO2 catalysts for the hydrogenation of crotonaldehyde on the hydrogen pressure, the reaction temperature, the nature of the solvent and the presence of several additives were investigated. Strong deactivation of the catalysts mainly caused by decarbonylation of crotonaldehyde and irreversible adsorption of CO was observed. The initial activity of the deactivated catalysts is regained by oxidation of the adsorbed CO to CO2 by purging with air. The selectivity to crotylalcohol increased with increasing number of turnovers per metal site. This is explained with preferential blocking of the nonselective sites on the metal crystallites by CO and by a slow surface modification of the aging catalyst with organic deposits. The overall rate and the selectivity to the saturated aldehyde are markedly enhanced by an increase in hydrogen pressure and by a decrease in reaction temperature. The addition of modifiers such as potassium acetate, triphenylphosphine and thiophene had only little influence on the activity and the selectivity of the catalysts

Transcriptional coupling of neighbouring genes and gene expression noise: evidence that gene orientation and non-coding transcripts are modulators of noise

For some genes, notably essential genes, expression when expression is needed is vital hence low noise in expression is favourable. For others noise is necessary for coping with stochasticity or for providing dice-like mechanisms to control cell fate. But how is noise in gene expression modulated? We hypothesise that gene orientation may be crucial, as for divergently organized gene pairs expression of one gene could affect chromatin of a neighbour thereby reducing noise. Transcription of antisense non-coding RNA from a shared promoter is similarly argued to be a noise-reduction mechanism. Stochastic simulation models confirm the expectation. The model correctly predicts: that protein coding genes with bi-promoter architecture, including those with a ncRNA partner, have lower noise than other genes; divergent gene pairs uniquely have correlated expression noise; distance between promoters predicts noise; ncRNA divergent transcripts are associated with genes that a priori would be under selection for low noise; essential genes reside in divergent orientation more than expected; bi-promoter pairs are rare subtelomerically, cluster together and are enriched in essential gene clusters. We conclude that gene orientation and transcription of ncRNAs, even if unstable, are candidate modulators of noise levels

Evidence of widespread degradation of gene control regions in hominid genomes

Although sequences containing regulatory elements located close to protein-coding genes are often only weakly conserved during evolution, comparisons of rodent genomes have implied that these sequences are subject to some selective constraints. Evolutionary conservation is particularly apparent upstream of coding sequences and in first introns, regions that are enriched for regulatory elements. By comparing the human and chimpanzee genomes, we show here that there is almost no evidence for conservation in these regions in hominids. Furthermore, we show that gene expression is diverging more rapidly in hominids than in murids per unit of neutral sequence divergence. By combining data on polymorphism levels in human noncoding DNA and the corresponding human¿chimpanzee divergence, we show that the proportion of adaptive substitutions in these regions in hominids is very low. It therefore seems likely that the lack of conservation and increased rate of gene expression divergence are caused by a reduction in the effectiveness of natural selection against deleterious mutations because of the low effective population sizes of hominids. This has resulted in the accumulation of a large number of deleterious mutations in sequences containing gene control elements and hence a widespread degradation of the genome during the evolution of humans and chimpanzees

Amino acid composition in endothermic vertebrates is biased in the same direction as in thermophilic prokaryotes

<p>Abstract</p> <p>Background</p> <p>Among bacteria and archaea, amino acid usage is correlated with habitat temperatures. In particular, protein surfaces in species thriving at higher temperatures appear to be enriched in amino acids that stabilize protein structure and depleted in amino acids that decrease thermostability. Does this observation reflect a causal relationship, or could the apparent trend be caused by phylogenetic relatedness among sampled organisms living at different temperatures? And do proteins from endothermic and exothermic vertebrates show similar differences?</p> <p>Results</p> <p>We find that the observed correlations between the frequencies of individual amino acids and prokaryotic habitat temperature are strongly influenced by evolutionary relatedness between the species analysed; however, a proteome-wide bias towards increased thermostability remains after controlling for phylogeny. Do eukaryotes show similar effects of thermal adaptation? A small shift of amino acid usage in the expected direction is observed in endothermic ('warm-blooded') mammals and chicken compared to ectothermic ('cold-blooded') vertebrates with lower body temperatures; this shift is not simply explained by nucleotide usage biases.</p> <p>Conclusion</p> <p>Protein homologs operating at different temperatures have different amino acid composition, both in prokaryotes and in vertebrates. Thus, during the transition from ectothermic to endothermic life styles, the ancestors of mammals and of birds may have experienced weak genome-wide positive selection to increase the thermostability of their proteins.</p