We have previously shown that Mu restores an active DNA replication at non-permissive temperature in E. coli K12 ligts7 strains. In this paper we describe two new types of phage mutants for the Mu lig function. The Mu ligts mutants are conditional lethals defective for both integration and replication of DNA, unable to 'complement' the bacterial lig mutation; the map between B and lys. The mutants of the other type, on the other hand, are able to restore to a maximum level the activity impaired by the ligts7 mutation in the host. We suggest the hypothesis that the product of Mu lig gene could be part of a complex as a topoisomerase.
Mu ligts2 mutants, defective for development and integration, show a high killing effect on the infected host. A number of survivors to Mu ligts2 infection were analyzed; they are characterized by nonpermissivity for both development and lysogenization of bacteriophage Mu. Bacteriophages D108 and P1 are also inhibited in these strains as is transposon Tn9. The corresponding mutation site was mapped at 82 min and identified with the Escherichia coli gyrB site.
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