title: Mutational and selective pressures on codon and amino acid usage in Buchnera, endosymbiotic bacteria of aphids.
abstract: We have explored compositional variation at synonymous (codon usage) and nonsynonymous (amino acid usage) positions in three complete genomes of Buchnera, endosymbiotic bacteria of aphids, and also in their orthologs in Escherichia coli, a close free-living relative. We sought to discriminate genes of variable expression levels in order to weigh the relative contributions of mutational bias and selection in the genomic changes following symbiosis. We identified clear strand asymmetries, distribution biases (putative high-expression genes were found more often on the leading strand), and a residual slight codon bias within each strand. Amino acid usage was strongly biased in putative high-expression genes, characterized by avoidance of aromatic amino acids, but above all by greater conservation and resistance to AT enrichment. Despite the almost complete loss of codon bias and heavy mutational pressure, selective forces are still strong at nonsynonymous sites of a fraction of the genome. However, Buchnera from Baizongia pistaciae appears to have suffered a stronger symbiotic syndrome than the two other species.
authors.affiliation: UMR BIO3P, Institut National de la Recherche Agronomique, BP35327, 35653 Le Rheu cedex, France. rispe@rennes.inra.fr
authors.forename: Claude
authors.initials: C
authors.lastname: Rispe
authors.forename: François
authors.initials: F
authors.lastname: Delmotte
authors.forename: Roeland C H J
authors.initials: RC
authors.lastname: van Ham
authors.forename: Andres
authors.initials: A
authors.lastname: Moya
mesh_terms: D000596:Amino Acids / Q000235:genetics*; D000818:Animals; D001042:Aphids / Q000382:microbiology*; D001482:Base Composition / Q000235:genetics; D020635:Buchnera / Q000235:genetics* / Q000378:metabolism*; D003062:Codon / Q000235:genetics*; D004926:Escherichia coli / Q000235:genetics; D019143:Evolution, Molecular; D005798:Genes, Bacterial / Q000235:genetics; D017344:Genes, Insect / Q000235:genetics; D016680:Genome, Bacterial; D008957:Models, Genetic; D016296:Mutagenesis; D012641:Selection, Genetic; D013045:Species Specificity; D013559:Symbiosis / Q000235:genetics*
chemical_list: D000596:Amino Acids; D003062:Codon