[Constitutive inhibition of DNA degradation due to the enzyme RecBCD in the radiation-resistant Escherichia coli K-12 mutant Gam(r)444].

: Exonucleolytic degradation of [3]H-labeled DNA was examined in partially purified fractions of lysates obtained from nonirradiated RecBCD enzyme-containing cells of Escherichia coli and in the radiation-resistant mutant Gamr444. The degradative activity was shown to be lowered in these cells to the same extent as in the recBC mutant. The efficiency of plating of the mutant phage T4 2-, DNA of which can be degraded by exonuclease V, was 400-fold higher on the strain Gamr444 than on the wild-type strain AB1157. This value was shown to be only twice as low as that on the recB mutant or on the strain AB1157 carrying plasmid pGam26 with a radiation-resistance allele gam26 cloned from mutant Gamr444. The data obtained confirmed the hypothesis that the Gamr444 mutant contains a constitutive inhibitor of exonucleolytic activity of the RecBCD enzyme in nonirradiated cells. This inhibitor was shown to be encoded by the gam26 allele that had previously been mapped at 56.8 min of the E. coli chromosome. A possible mechanism of the involvement of this inhibitor in enhanced radiation resistance of the mutant Gamr444 is considered.