Genetic recombination between a nontandem duplication of two partially deleted lactose operons (lacMS286)8OdIlacBKl) in Escherichia coli K-12 has been examined. Since the deletions were nonoverlapping, rare lactose-fermenting (Lac+) recombinants occurred and were detected qualitatively on lactose tetrazolium agar indicator plates as white papillae growing on the surface of red colonies or quantitively on lactose minimal agar plates. Formation of Lac+ recombinants required the recA, recB, and

more »

... C gene products. Indirect suppression of recB21 by sbcB15 led to an increase in the frequency of Lac+ recombinants over wild-type levels. recF143 did not appreciably alter the number of Lac+ progeny, whereas recL152 and sbcB15 strains yielded increased numbers of Lac+ recombinants. The nature and formation of Lac+ recombinants was also examined. Respreading analysis indicated that formation of recombinants occurred primarily as the cells entered early stationary phase on the surface of the minimal agar plates and that over 90% of the recombinants contained a O80dIIlac+ prophage. Time-of-entry experiments suggested that the region of deoxyribonucleic acid between the two operons was not inverted as a result of the recombinational event. MATERIALS AND METHODS Materials. Reagents were obtained from the following sources: lactose, glucose-free, U.S. Biochemical Corp.; 2,3,5-triphenyl-2H-tetrazolium chloride, Eastman Kodak Co.; streptomycin sulfate and amino acids, Sigma Chemical Co.; H332PO4, carrierfree, New England Nuclear Corp. All other chemicals were of reagent grade. Bacterial strains and bacteriophage. Bacterial strains are listed in Table 1 . Nomenclature conforms to that of Demerec et al. (5) , except that a minus sign is used to describe a general mutant allele. Gene symbols are those used by Bachmann et al. (1) .