SYNONYMOUS CODON USAGE IN ZEA-MAYS L NUCLEAR GENES IS VARIED BY LEVELS OF C-ENDING AND G-ENDING CODONS

A multivariate statistical method called correspondence analysis was used to examine the codon usage of one-hundred-and-one nuclear genes of maize (Zea mays L.). Forty percent of the variation in codon usage was due to bias toward G or C-ending versus A or U-ending codons. Differences in levels of G-ending codons showed the weakest correlation with the major codon usage bias. The bias toward C or U versus A or G in the silent third nucleotide position of synonymous codons accounted for approximately 10% of the variation in codon usage. The G + C content of the silent third nucleotide position of coding regions was not strongly correlated with G + C content of introns. Codon usage was strongly biased toward codons ending in G or C for a number of highly expressed genes including most light-regulated chloroplast proteins, ABA-induced proteins, histones, and anthocyanin biosynthetic enzymes. Codon usage of genes encoding storage proteins and regulatory proteins, such as transposases, kinases, phosphatases and transcription factors, was more random than that of genes encoding cytosolic enzymes with similar blas toward G or C-ending codons. Codon usage in maize may reflect both regional bias on nucleotide composition and selection on the silent third nucleotide position.