Comparative analyses of base compositions, DNA sizes, and dinucleotide frequency profiles in archaeal and bacterial chromosomes and plasmids.

In the present paper, I compared guanine-cytosine (GC) contents, DNA sizes, and dinucleotide frequency profiles in 109 archaeal chromosomes, 59 archaeal plasmids, 1379 bacterial chromosomes, and 854 bacterial plasmids. In more than 80% of archaeal and bacterial plasmids, the GC content was lower than that of the host chromosome. Furthermore, most of the differences in GC content found between a plasmid and its host chromosome were less than 10%, and the GC content in plasmids and host chromosomes was highly correlated (Pearson's correlation coefficient r = 0.965 in bacteria and 0.917 in archaea). These results support the hypothesis that horizontal gene transfers have occurred frequently via plasmid distribution during evolution. GC content and chromosome size were more highly correlated in bacteria (r = 0.460) than in archaea (r = 0.195). Interestingly, there was a tendency for archaea with plasmids to have higher GC content in the chromosome and plasmid than those without plasmids. Thus, the dinucleotide frequency profile of the archaeal plasmids has a bias toward high GC content.