Evolution of DNA sequence : mechanisms and models as seen in small phages : (small isometric phages, evolution of their nucleotide sequence, duplication, deletion, overlapping genes, secondary structure)

Extensive nucleotide sequence data is new known for six small isometric phages, ([o with diagonal slash]Xl 74, G4, St-1, Sl3, [lowercase alpha]3 and [phi]K).Comparison of these sequences allows us to observe the role of deletion, duplication, and nucleotide drift in the evolution of these phages. It is evident that small deletions play a considerable role in the changes observed in both the coding and untranslated, non-coding regions of these phages. There are no deletions in the overlapping gene regions, but surprisingly it is found that the rate of nucleotide change in the overlapping genes is very little different from that observed in the non-overlapping genes. The dnaG primase recognition site, the origin of complementary DNA strand synthesis, is present in different locations in these genomes, suggesting that it might have been acquired from an outside source, sometime in the past.G. NIGEL GODSON, Radiobiology Laboratories, Yale Medical School