Strong heterogeneity in nucleotidic composition and codon bias in the pea aphid (Acyrthosiphon pisum) shown by EST-based coding genome reconstruction.

The aim of this study was to analyze patterns of nucleotidic composition and codon usage in the pea aphid genome (Acyrthosiphon pisum). A collection of 60,000 expressed sequence tags (ESTs) in the pea aphid has been used to automatically reconstruct 5809 coding sequences (CDSs), based on similarity with known proteins and on coding style recognition. Reconstructions were manually checked for ribosomal proteins, leading to tentatively reconstruct the nea-complete set of this category. Pea aphid coding sequences showed a shift toward AT (especially at the third codon position) compared to drosophila homologues. Genes with a putative high level of expression (ribosomal and other genes with high EST support) remained more GC3-rich and had a distinct codon usage from bulk sequences: they exhibited a preference for C-ending codons and CGT (for arginine), which thus appeared optimal for translation. However, the discrimination was not as strong as in drosophila, suggesting a reduced degree of translational selection. The space of variation in codon usage for A. pisum appeared to be larger than in drosophila, with a substantial fraction of genes that remained GC3-rich. Some of those (in particular some structural proteins) also showed high levels of codon bias and a very strong preference for C-ending codons, which could be explained either by strong translational selection or by other mechanisms. Finally, genomic traces were analyzed to build 206 fragments containing a full CDS, which allowed studying the correlations between GC contents of coding and those of noncoding (flanking and introns) sequences.