Evolution of codon usage and base contents in kinetoplastid protozoans.

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Abstract
In this study we analyze and compare the trends in codon usage in five representative species of kinetoplastid protozoans (Crithidia fasciculata, Leishmania donovani, L. major, Trypanosoma cruzi and T. brucei), with the purpose of investigating the processes underlying these trends. A principal component analysis shows that the G+C content at the third codon position represents the main source of codon-usage variation, both within species (among genes) and among species. The non-Trypanosoma species exhibit narrow distributions in codon usage, while both Trypanosoma species present large within-species heterogeneity. The three non-Trypanosoma species have very similar codon-usage preferences. These codon preferences are also shared by the highly expressed genes of T. cruzi and to a lesser degree by those of T. brucei. This leads to the conclusion that the codon preferences shared by these species are the ancestral ones in the kinetoplastids. On the other hand, the study of noncoding sequences shows that Trypanosoma species exhibit mutational biases toward A + T richness, while the non-Trypanosoma species present mutational pressure in the opposite direction. These data taken together allow us to infer the origin of the different codon-usage distributions observed in the five species studied. In C. fasciculata and Leishmania, both mutational biases and (translational) selection pull toward G + C richness, resulting in a narrow distribution. In Trypanosoma species the mutational pressure toward A + T richness produced a shift in their genomes that differentially affected coding and noncoding sequences. The effect of these pressures on the third codon position of genes seems to have been inversely proportional to the level of gene expression.

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