The relative contribution of mutation and selection to the G+C content of DNA 
was analyzed in bacterial species having widely different G+C contents. The 
analysis used two methods that were developed previously. The first method was 
to plot the average G+C content of a set of nucleotides against the G+C content 
of the third codon position for each gene. This method was used to present the 
G+C distribution of the third codon position and to assess the relative 
neutrality of a set of nucleotides to that of the G+C content of the third codon 
position. The second method was to plot the intrastrand bias of the third codon 
position from Parity Rule 2 (PR2), where A = T and G = C. It was found that 
whereas intragenomic distributions of the DNA G+C content of these bacteria are 
narrow in the majority of species, in some species the G+C content of the minor 
class of genes distributes over wider ranges than the major class of genes. On 
the other hand, ubiquitous PR2 biases are amino acid specific and independent of 
the G+C content of DNA, so that when averaged over the amino acids, the biases 
are small and not correlated with the DNA G+C content. Therefore, translation 
coupled PR2-biases are unlikely to explain the wide range of G+C contents among 
different species. Considering all data available, it was concluded that the 
amino acid-specific PR2 bias has only a minor effect, if any, on the average G+C 
content. In addition, PR2 bias patterns of different species show phylogenetic 
relationships, and the pattern can be as a taxal fingerprint.