The hsd (host specificity) genes of E. coli K12

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Abstract
The hsd genes of E. coli K12 have been cloned in phage λ by a combination of in vitro and in vivo techniques. Three genes, whose products are required for K-specific restriction and modification, have been identified by complementation tests as hsdR, M, and S. The order of these closely linked genes was established as R, M, S by analysis of the DNA of genetically characterised deletion derivatives of λhsd phages. The three genes are transcribed in the same direction but not necessarily as a single operon. Genetic evidence identifies two promoters, one from which transcription of hsdM and S is initiated and a second for the hsdR gene. The hsdR gene codes for a polypeptide of molecular weight ∼130000; hsdM for one of 62–65000 and the hsdS gene was associated with two polypeptides of approximately 50000. Circumstantial evidence suggest that one of these two polypeptides may be a degradation, or processed, derivative of the other. The hsdS polypeptide of E. coli B has a slightly higher mobility in an SDS-polyacrylamide gel than does that of E. coli K 12. A probe comprising most of the hsdR gene and all of the hsdM and S genes of E. coli K 12 shares extensive homology with the DNA of E. coli B but none with that of E. coli C.