Codon usage bias in 5' terminal coding sequences reveals distinct enrichment of gene functions.

Codon bias at the 5' terminal of coding sequence (CDS) is known to be distinct from the rest of the CDS. A number of events occur in this short region to regulate early translation elongation and co-translational translocation. In the genes encoding secretory proteins, there is a special signal sequence which has a higher occurrence of rare codons. In this study, we analyzed codon bias of secretory genes in several eukaryotes. The results showed that secretory genes in the species except mammals had a higher occurrence of rare codons in the 5' terminal of CDS, and the bias was greater than the same region of non-secretory genes. GO analysis revealed that secretory genes containing rare codon clusters in different regions were responsible for various roles in gene functions. Moreover, codon bias in the region encoding the hydrophobic region of protein is similar in secretory and non-secretory genes, indicating that codon bias in secretory genes was partly influenced by amino acid bias. Rare codon clusters are found more frequently in specific regions, and continuous rare codons are not favoured probably because they will increase the probability of ribosome collision and drop-off. Based on ribosome profiling data, there is no significant difference in the average translation efficiencies between rare and optimal codons. Higher ribosomal density in the 5' terminal may result from ribosome pausing which could be involved in different translation events. These findings collectively provided rich information on codon bias in secretory genes, which may shed light on the co-effect of codon bias, mRNA structure and tRNA abundance in translational regulations.