Functional Analysis of Second-step Host Specificity Mutations in Unstable Escherichia coli Heterozygotes

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Abstract
SUMMARY From an Escherichia cofi strain KI 2 carrying a temperature-sensitive host- specific modification (hsrn) 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 tem- perature-sensitive and the second-step which impairs restriction and modification completely, are located in hsrn. Analysis of the heterozygotes used in the com- plementation 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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