Comparative study on factors influencing the codon and amino acid usage in Lactobacillus sakei 23K and 13 other lactobacilli

Genome-wide analysis of the synonymous codon usage pattern of Streptococcus suis

Streptococcus suis (S. suis) is a gram-positive coccus that causes disease in humans and animals. The codon usage pattern of bacteria reveals a range of evolutionary changes that assist them to enhance tolerance to environments. To better understand the genetic features during the evolution of S. suis, we performed codon usage analysis. Nine pathogenic strains of different serotypes and different geographical distribution were analyzed to better understand the differences in their evolutionary process. Nucleotide compositions and relative synonymous codon usage (RSCU) analysis revealed that A/T-ending codons are dominant in S. suis. Neutrality analysis, correspondence analysis and ENC-plot results revealed that natural selection is the predominant element prompting codon usage. Cluster analysis based on RSCU was roughly consistent with the dendrogram rooted genomic BLAST analysis. Comparison of synonymous codon usage pattern between S. suis and susceptible hosts (H. sapiens and S. scrofa) revealed that the codon usage of S. suis is separated from the synonymous codon usage of susceptible hosts. The CAI values implied that S. suis includes a series of predicted highly expressed coding sequences contained in metabolism and transcriptional regulation, revealing the necessity of this pathogen to deal with various environmental conditions. The study of codon usage in S. suis may provide evidence involving the molecular evolution of bacteria and a better understanding of evolutionary relationships between S. suis and its corresponding hosts.

Switches in Genomic GC Content Drive Shifts of Optimal Codons under Sustained Selection on Synonymous Sites

The major codon preference model suggests that codons read by tRNAs in high concentrations are preferentially utilized in highly expressed genes. However, the identity of the optimal codons differs between species although the forces driving such changes are poorly understood. We suggest that these questions can be tackled by placing codon usage studies in a phylogenetic framework and that bacterial genomes with extreme nucleotide composition biases provide informative model systems. Switches in the background substitution biases from GC to AT have occurred in Gardnerella vaginalis (GC = 32%), and from AT to GC in Lactobacillus delbrueckii (GC = 62%) and Lactobacillus fermentum (GC = 63%). We show that despite the large effects on codon usage patterns by these switches, all three species evolve under selection on synonymous sites. In G. vaginalis, the dramatic codon frequency changes coincide with shifts of optimal codons. In contrast, the optimal codons have not shifted in the two Lactobacillus genomes despite an increased fraction of GC-ending codons. We suggest that all three species are in different phases of an on-going shift of optimal codons, and attribute the difference to a stronger background substitution bias and/or longer time since the switch in G. vaginalis. We show that comparative and correlative methods for optimal codon identification yield conflicting results for genomes in flux and discuss possible reasons for the mispredictions. We conclude that switches in the direction of the background substitution biases can drive major shifts in codon preference patterns even under sustained selection on synonymous codon sites.

Genome-Wide Analysis of the Synonymous Codon Usage Patterns in Riemerella anatipestifer

Riemerella anatipestifer (RA) belongs to the Flavobacteriaceae family and can cause a septicemia disease in poultry. The synonymous codon usage patterns of bacteria reflect a series of evolutionary changes that enable bacteria to improve tolerance of the various environments. We detailed the codon usage patterns of RA isolates from the available 12 sequenced genomes by multiple codon and statistical analysis. Nucleotide compositions and relative synonymous codon usage (RSCU) analysis revealed that A or U ending codons are predominant in RA. Neutrality analysis found no significant correlation between GC12 and GC₃ (p > 0.05). Correspondence analysis and ENc-plot results showed that natural selection dominated over mutation in the codon usage bias. The tree of cluster analysis based on RSCU was concordant with dendrogram based on genomic BLAST by neighbor-joining method. By comparative analysis, about 50 highly expressed genes that were orthologs across all 12 strains were found in the top 5% of high CAI value. Based on these CAI values, we infer that RA contains a number of predicted highly expressed coding sequences, involved in transcriptional regulation and metabolism, reflecting their requirement for dealing with diverse environmental conditions. These results provide some useful information on the mechanisms that contribute to codon usage bias and evolution of RA.

Analysis of synonymous codon usage patterns in the genus Rhizobium

The codon usage patterns of rhizobia have received increasing attention. However, little information is available regarding the conserved features of the codon usage patterns in a typical rhizobial genus. The codon usage patterns of six completely sequenced strains belonging to the genus Rhizobium were analysed as model rhizobia in the present study. The relative neutrality plot showed that selection pressure played a role in codon usage in the genus Rhizobium. Spearman's rank correlation analysis combined with correspondence analysis (COA) showed that the codon adaptation index and the effective number of codons (ENC) had strong correlation with the first axis of the COA, which indicated the important role of gene expression level and the ENC in the codon usage patterns in this genus. The relative synonymous codon usage of Cys codons had the strongest correlation with the second axis of the COA. Accordingly, the usage of Cys codons was another important factor that shaped the codon usage patterns in Rhizobium genomes and was a conserved feature of the genus. Moreover, the comparison of codon usage between highly and lowly expressed genes showed that 20 unique preferred codons were shared among Rhizobium genomes, revealing another conserved feature of the genus. This is the first report of the codon usage patterns in the genus Rhizobium.