Endonuclease from Acholeplasma laidlawii strain JA1 associated with in vivo restriction of DNA containing 5-methylcytosine

Transfection of Acholeplasma laidlawii strain JA1 with viral DNAs that have been sequence-specifically methylated in vitro has led us to previously postulate that JA1 cells contain an enzyme which cleaves only DNA containing 5-methylcytosine, regardless of the nucleotide sequence containing that base. In this paper we show that an endonuclease activity is present in extracts from JA1 cells but not in extracts from a restriction-deficient variant of JA1. The partially purified enzyme cleaves not only DNA containing 5-methylcytosine, but also DNA containing no methylated bases. We discuss why this endonuclease activity may not have the same in vitro specificity as would be expected from in vivo experiments.