DNA methylation of T3 virus ocr+ and ocr- strains in Escherichia coli cells harbouring the EcoK DNA host specificity system.

The influence of the T3 gene functions ocr+ and sam+ on the extent of phage DNA methylation in Escherichia coli K12 cells was studied by determining the proportion of 6- methylaminopurin to adenine in the purified DNA of T3 wild-type, sam- and ocr- sam- phage strains. We demonstrate that the DNA of T3 ocr- sam- mutants carries 12 methyl groups as a result of the action of the host-specificity methylase EcoK . In contrast to this the DNA of ocr+ strains is not EcoK -specifically methylated.