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Proc Natl Acad Sci U S A. 1986 Dec;83(23):9070-4.

Escherichia coli K-12 restricts DNA containing 5-methylcytosine.


We have observed that plasmids containing certain cloned modification methylase genes of type II restriction-modification systems cannot be transformed into many laboratory strains of Escherichia coli K-12. The investigation of this phenomenon, reported here, has revealed (i) DNA containing 5-methylcytosine is biologically restricted by these strains, while DNA containing 6-methyladenine is not; (ii) restriction is due to two genetically distinct systems that differ in their sequence specificities, which we have named mcrA and mcrB (for modified cytosine restriction). Since 5-methylcytosine containing DNA is widespread in nature, the Mcr systems probably have a broad biological role. Mcr restriction may seriously interfere with molecular cloning of 5-methylcytosine-containing foreign DNAs. The Mcr phenotypes of some commonly used strains of E. coli K-12 are reported.

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