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.