The frequency of oligopurine. Oligopyrimidine and other two‐base tracts in yeast chromosome III

The TRACTS program was employed to map the occurrence of base tracts composed of only two bases in Saccharomyces cerevisiae chromosome III. The observed frequencies were compared with those expected in random DNA. A vast excess of long base tracts of the three possible two-base combinations, namely, purine.pyrimidine (R.Y), keto.imino (K.M) and weak;strong (W;S, mainly A,T rich), was documented. The observed excess places yeast in the same category as other eukaryote and organelle genomes analysed. The excess of the two-base tracts was considerably larger in the 1/3 of the chromosome not coding for a protein, in particular proximal to coding initiation and termination sites, but was observed for coding regions as well. A functional role for the excessive tracts, possibly as unwinding centers of particular genes, is proposed. Multiple occurrence of long two-base tracts is offered as another diagnostic to determine whether an open reading frame (ORF), or an ORF subregion, is an actually translated gene region.