Abstract
A system for detecting a spontaneous deletion in Escherichia coli was developed and the role of DNA gyrase in deletion formation was studied. A derivative of lambda plac5, lambda AM36, was isolated in which whole pBR322 DNA was inserted in the lacZ gene and 227 bp of the lac gene duplicated at both sides of the pBR322 DNA. E. coli lac- strains lysogenized by lambda AM36 had a Lac- phenotype and segregated Lac+ revertants. Sequence analyses showed that the revertant was formed by a deletion that eliminated the inserted pBR322 DNA and one copy of the duplicated segments. The frequency of lac+ revertant formation was independent of recA function, was increased by oxolinic acid, an inhibitor of DNA gyrase, but was not increased in a lysogen of a nalidixic acid-resistant derivative. The reversion frequencies of temperature sensitive mutants of gyrA gene are 10 to 100 times lower than that of the wild-type strain. These results indicate that the DNA gyrase of E. coli participated in the in vivo deletion formation resulting from the direct repeats.
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