Compositional bias coupled with selection and mutation pressure drives codon usage in Brassica campestris genes

The plant Brassica campestris includes the vegetables turnip and Chinese cabbage, important plants of economic importance. Here, we have analysed the codon usage bias of B. campestris for 116 protein coding genes. Neutrality analysis showed that B. campestris had a wide range of GC3s, and a significant correlation was observed between GC12 and GC3. Nc versus GC3s plot showed a few genes on or proximate to the expected curve, but the majority of points were found to be scattered distantly from the expected curve. Correspondence analysis on codon usage revealed that the position preference of codons on multidimensional space totally depends on the presence of A and T at synonymous third codon position. These results altogether suggest that composition bias along with selection (major) and mutation pressure (minor) affects the codon usage pattern of the protein coding genes in Brassica campestris.