[Mutant alleles for radioresistance form the Escherichia coli strain Gam(r)444): cloning and preliminary characteristics].

Mutant alleles Gamr, which are able to increase the resistance to radiation of Escherichia coli wild-type cells, were cloned from the hyperradioresistant mutant Gamr444 on a plasmid mini-Mu vector MudII4042. The influence of recombinant plasmids on the sensitivity of wild-type and mutant (recA and htpR) cells to gamma-irradiation was studied. It was shown that the enhanced resistance of the Gamr444 strain to radiation was caused by mutations of two different classes, dominant and recessive. The cloned recessive mutation gamr12 increases resistance to radiation only after homogenization, that is, radiation-induced transfer from the plasmid to the chromosome, and it imposes constitutive expression of the heat-shock promoter htpG. Dominant mutant gamr alleles are active in the trans-position. A mutation-insertion into a chromosomal gene impaired by one of the dominant mutations, gamr18, was constructed. The insertion causes drastic cell radiosensitization on the recB sbcB background and probably disturbs the RecF pathway of recombination and repair. Dominant plasmids of the second type lead to the RecA-independent inhibition of DNA postirradiation degradation. The radioprotective action of recessive and dominant gamr mutations is additive.