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Eight recombination-deficient (Rec− ) mutants ofEscherichia coli were studied. Progeny lines were obtained on solid media, by means of micromanipulation, and the colony-forming ability of individual cells was analyzed. Cells of all eight strains gave rise to colony-forming as well as non-colony-forming descendants (“lethal sectoring”). Lethal sectors, i.e., groups of non-colony-forming cells which originate from a common ancestor, appeared with frequencies per generation ranging between 4 and 20% in Rec− strains, whereas lethal sectors were rare in Rec+ strains (less than 1%). A strain carrying a mutation (uvrA6 ) in one of the genes involved in pyrimidine dimer excision from deoxyribonucleic acid (DNA) showed twice as many lethal sectors per generation as a strain with the genotypeuvrA + . Similarly, a double mutant (AB2480,uvrA6, recA13 ) showed twice as much spontaneous lethal sectoring as the corresponding Rec− strain (uvrA + ,recA13 ). The kinetics of growth curves obtained in nutrient broth and the frequency of non-colony-forming units in stationary-phase broth cultures indicate clearly that lethal sectors occur in liquid cultures too. The causes for spontaneous lethal sectoring are unknown at present. It seems reasonable to assume that geneuvrA and therec genes are somehow involved in the repair of spontaneously occurring DNA lesions, since a deficiency in this type of repair may cause lethal sectors. The extent to which spontaneous lethal sectoring (observed in all Rec− strains ofE. coli studied) may contribute indirectly to the failure to form recombinants is discussed.

Spontaneous Lethal Sectoring, a Further Feature of Escherichia coli Strains Deficient in the Function of rec and uvr Genes