MUTANT RADIATION-RESISTANCE ALLELES FROM THE ESCHERICHIA-COLI GAM(R)444 MUTANT - CLONING AND PRELIMINARY CHARACTERIZATION

Mutant alleles Gam(r), which are able to increase the resistance to ra diation of Escherichia coli wild-type cells, were cloned from the hype rradioresistant mutant Gam(r)444 on a plasmid mini-Mu vector MudII4042 . The influence of recombinant plasmids on the sensitivity of wild-typ e and mutant (recA and htpR) cells to gamma-irradiation was studied. I t was shown that the enhanced resistance of the Gam(r)444 strain to ra diation was caused by mutations of two different classes, dominant and recessive. The cloned recessive mutation gam(r)12 increases resistanc e to radiation only after homogenotization, that is, radiation-induced transfer from the plasmid to the chromosome, and it imposes constitut ive expression of the heat-shock promoter htpG. Dominant mutant gam(r) alleles are active in the trans-position. A mutation-insertion into a chromosomal gene impaired by one of the dominant mutations, gam(r)18, 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 t o the RecA-independent inhibition of DNA postirradiation degradation. The radioprotective action of recessive and dominant gam(r) mutations is additive.