Phenotypically cryptic EcoRI endonuclease activity specified by the ColE1 plasmid.

An endonuclease having EcoRI specificity is produced by bacteria containing the ColE1 plasmid. Such bacterial cells fail to express restriction or modification functions in vivo, and phage or plasmid DNA obtained from ColE1-containing cells has unmodified EcoRI sites that are cleaved in vitro by purified EcoRI endonuclease or by enzyme extracted from bacteria that carry ColE1. No EcoRI DNA methylase activity associated with ColE1 has been detected. The finding of phenotypically cryptic ColE1-dependent EcoRI endonuclease activity and the absence of any detectable EcoRI modification system in ColE1-containing cells suggest a control mechanism that appears to prevent functional expression of the ColE1-determined enzyme in vivo.