HOST-CONTROLLED RESTRICTION AND MODIFICTAION OF SALMONELLA TYPHIMURIUM

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The poor recipient ability of Salmonella typhimurium LT-2 for the transfer of genetic material from Escherichia coli K-12 was found to be due to host-con-trolled restriction and modification. Rfer 1 mutants of LT-2 with increased re-cipient ability were shown to be deficient in restriction and modification. Rfer 2 mutants, which were isolated from Rfer 1 and which had even better recipient ability than Rfer 1, were found to have rough mutations in addition to the restric-tion and modification deficiency. Anomalous characters of ƒi- R factors with regard to restriction and modification are also reported.