ИСТИНА

Restriction-modification (R-M) systems defend prokaryotes from the invasion of foreign DNA by recognition and cleavage of specific DNA sequences. The R-M systems of Type II induce avoidance of their recognition sites in genomes of prokaryotes and their viruses [1]. To estimate site avoidance, we implemented an algorithm suggested in [2, 3]. We analyzed capabilities of site avoidance estimation for prediction of host specificity of prokaryotic viruses. The site is called avoided in a genome if the expected number of sites is more than 10 and observed number of sites is less than 1/5 of the expected number. It was found that haloviruses of Myoviridae family, which are known to have broad host ranges [4], avoid several dozens of sites in their genomes among 370 known Type II R-M systems sites. Probably site avoidance is the main antirestriction strategy of these viruses. To verify this hypothesis, we compared avoided sites in 5 known Myoviridae haloviruses and known 30 sites of orthodox Type II RM systems of different haloarchaea. About 50% of avoided sites belong to any haloarchaeal Type II R-M system. This rate is significantly higher than it could be found by chance, taken into account the low fraction of haloarchaeal Type II sites in the entire set (30 of 370).