Molecular Biology and Evolution, Vol 11, 790-802, Copyright © 1994 by Society for Molecular Biology and Evolution
F Alvarez, C Robello and M Vignali
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.
ORIGINAL ARTICLE
Evolution of codon usage and base contents in kinetoplastid protozoans
Departamento de Genetica, Facultad de Medicina, Montevideo, Uruguay.
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