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Abstract
SUMMARY: From an Escherichia coli strain k12 carrying a temperature-sensitive hostspecific modification (hsm) mutation, second-step mutants have been isolated that are completely deficient in modification and restriction. Complementation analysis has revealed that one group of these mutants is impaired in the specificity gene hss, while in the other group of mutant strains both mutations, i.e. the first-step temperature-sensitive and the second-step which impairs restriction and modification completely, are located in hsm. Analysis of the heterozygotes used in the complementation experiments suggested a cis- and tandem arrangement of the hs and leu genes in haploid, segregating exconjugants; however, the attachment of these genes to a cryptic plasmid was not excluded.
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