Summary: An K12 chromosomal RI-HI fragment containing a mutant locus was cloned into plasmid pBR322. The gene, closely linked to , was used for selection of clones with the inserted fragment using 14 and λII phages; the phage DNAs contain methylated cytosines and hence can be used to demonstrate McrB restriction. For the efficient expression of the gene, a II fragment of phage carrying the p promoter was inserted into the HI site of the hybrid plasmid. Under these conditions a effect of the mutation on restriction and modification was detected. Inactivation of the gene by the insertion of the phage II fragment into the II site within this gene resulted in the disappearance of the -dominant effect. When the cloned HI-RI fragment was shortened by I and RI restriction enzymes, the -dominant effect was fully expressed. The results indicate that the mutation is located in the S gene. The effect of gene dosage of the HsdS subunit on the expression of mutation was studied. The results of complementation experiments, using F'-merodiploids or plasmid pBR322 with an inserted mutation, support the idea that the HsdS product competes with the wild-type HsdS product, and has a quantitatively different effect on restriction and modification.


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