Host specificity of Salmonella typhimurium deoxyribonucleic acid restriction and modification.

The restriction and modification genes of Salmonella typhimurium which lie near the thr locus were transferred to a restrictionless mutant of Escherichia coli. These genes were found to be allelic to the E. coli K, B, and A restriction and modification genes. E. coli recombinants with the restriction and modification host specificity of S. typhimurium restricted phage lambda that had been modified by each of the seven known host specificities of E. coli at efficiency of plating levels of about 10(-2). Phage lambda modified with the S. typhimurium host specificity was restricted by six of the seven E. coli host specificities but not by the RII (fi(-) R-factor controlled) host specificity. It is proposed that the restriction and modification enzymes of this S. typhimurium host specificity have two substrates, one of which is a substrate for the RII host specificity enzymes.