Codon usage in selected AT-rich bacteria.

The relationship between DNA base composition and codon bias in very AT-rich bacteria was analyzed. Five clostridial genes, five mycoplasmal genes and three rickettsial genes constituted the data base. In the genes of these three organisms, the rule for codon bias was very simple: use U or A in the first and third positions of the codon when possible. This was contrasted with the bias found in Bacillus subtilis and Escherichia coli. The rule for Bacillus subtilis was equally straightforward: use all codons without bias. Only in E. coli, amongst the species examined, did the codon bias appear to be a complicated codon 'choice'.