SUMMARY: From an strain carrying a temperature-sensitive hostspecific modification () 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 , 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 Analysis of the heterozygotes used in the complementation experiments suggested a - and tandem arrangement of the and genes in haploid, segregating exconjugants; however, the attachment of these genes to a cryptic plasmid was not excluded.


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