Tracts of adenosine and cytidine residues in the genomes of prokaryotes and eukaryotes

Large segments of the S. cerevisiae, C. elegans, D. melanogaster, mouse, and human genomes, as well as the genomes of four bacterial species, have been analyzed for the occurrence of tracts of separated, alternating, and mixed adenosine and cytidine residues. Several surprising features have been observed. Although both yeast and nematode DNA are rich in AT base pairs, the genomes of these organisms have widely different biases for long homonucleotide tracts. Yeast has many long tracts of oligoadenosine, while C. elegans has an extraordinary abundance of oligocytidine tracts. Tracts of alternating A-C residues are overrepresented in most eukaryotic organisms examined. Tracts of mixed adenosine and cytidine residues, however, are especially frequent in the human genome.