PNNL

Abstract

SUMMARY-Highly expressed genes in bacteria often have a stronger codon bias than genes expressed at lower levels. In this study, a comparative analysis of predicted highly expressed (PHX) genes in the Streptomyces coelicolor and S. avermitilis genomes was performed using the codon adaptation index (CAI) as a numerical estimator of gene expression level. Although it has been suggested that there is little heterogeneity in codon usage in G+C rich bacteria, considerable heterogeneity was found among genes in two G+C rich Streptomyces genomes. Using ribosomal protein (RP) genes as references, ~10% of the genes were predicted to be PHX genes using a CAI cutoff value of greater than 0.78 and 0.75 in S. coelicolor and S. avermitilis, respectively. Most of the PHX genes were found to be located within the conserved cores of the Streptomyces linear chromosomes. The predicted PHX genes showed good agreement with the experimental data on expression levels collected by proteomic analysis (Hesketh et al., 2002). Among all PHX genes, 368 were conserved in both genomes. These represented most of the genes essential for cell growth, including those involved in protein and DNA biosynthesis, amino acid metabolism, central intermediary and energy metabolisms. Only a few genes directly involved in biosynthesis of secondary metabolites were predicted to be PHX genes. Correspondence analysis showed that the genes responsible for biosynthesis of secondary metabolites possessed different codon usage patterns from RP genes, suggesting that they were either under strong translational selection that may have driven the codon preference in another direction, or they were acquired by horizontal transfer during their origin and evolution. Nevertheless, several key genes responsible for producing precursors for secondary metabolites, such as crotonyl-CoA reductase and propionyl-CoA carboxylase, and genes necessary for initiation of secondary metabolism, such as adenosylmethionine synthetase were among the PHX genes in two Streptomyces. The unique PHX genes in each genome, and what they imply regarding primary and secondary metabolism in these species are also discussed.