CodonO: a new informatics method for measuring synonymous codon usage bias within and across genomes

Codon usage bias reveals the silent molecular evolution pattern. Previous research showed the codon usage bias was associated with many biological processes, such as protein expression level, genetic coding error minimization, mRNA stability, codon context, tRNA richness, CpG suppression, DNA methylation, and tissue or organ specificity. In this paper we reviewed major methods of codon usage bias measurement. Since most of existing methods are only suitable for the comparison of codon usage bias within a single genome, we introduced a new informatics index, referred to as synonymous codon usage order (SCUO), to measure synonymous codon usage bias within and across genomes. In this method, Shannon informational theory was applied to describe the SCUO of each gene using a value ranging from 0 to 1, with larger values associated with greater codon usage bias. We compared our method with the codon adaptation index (CAI) method for measuring codon bias in Escherichia coli and Sacharomyces cerevisiae. We also studied the correlation between SCUO and CAI, and the relation of SCUO with gene length, gene function, and GC composition. In addition, we explored the correlation between SCUO and mRNA abundance in S. cerevisiae using SAGE expression data. The software package codonO is freely available at http://digbio.missouri.edu/～wanx/cu/codonO/.