Cloning and structural characterization of the mcrA locus of Escherichia coli.

Escherichia coli has DNA restriction systems which are able to recognize and attack modified cytosine residues in the DNA of incoming bacteriophages and plasmids. The locus for the McrA/RglA system of modified cytosine restriction was located near the pin gene of the defective element, e14. Hence, loss of the e14 element through abortive induction after UV irradiation caused a permanent loss of McrA restriction activity. e14 DNA encoding McrA restriction was cloned and sequenced to reveal a single open reading frame of 831 bp with a predicted gene product of 31 kDa. Clones expressing the complete open reading frame conferred both McrA and RglA phenotypes; however, a deletion derivative was found which complemented RglA restriction against nonglucosylated T6gt phage but did not complement for McrA restriction of methylated plasmid DNA. Possible explanations for this activity and a comparison with the different organization of the McrB/RglB restriction system are discussed.