Restriction alleviation and modification enhancement by the Rac prophage of Escherichia coli K-12.

Bacteriophage lambda encodes an antirestriction function, RaI, which is able to modulate the activity of the Escherichia coli K-12 restriction and modification system, EcoKI. Here we report the characterization of an analogous function, Lar, expressed by E. coli sbcA mutants and the hybrid phage lambda reverse. E. coli sbcA mutants and lambda reverse both express genes of the Rac prophage, and we have located the lar gene immediately downstream of recT in this element. The lar gene has been cloned in an expression plasmid, and a combination of site-directed mutagenesis and labelling of plasmid-encoded proteins has enabled us to identify a number of translational products of lar, the smallest of which is sufficient for restriction alleviation. Lar, like RaI, is able both to alleviate restriction and to enhance modification by EcoKI. Lar, therefore, is functionally similar to RaI and the nucleotide sequences of their genes share 47% identity, indicating a common origin. A comparison of the predicted amino acid sequences of Lar and RaI shows only a 25% identity, but a few short regions do align and may indicate residues important for structure and/or function.