Escherichia coli K-12 Mutants Resistant to Nalidixic Acid: Genetic Mapping and Dominance Studies

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Abstract
Escherichia coli K-12 strains tested so far (approximately 20) can be separated into three groups on the basis of their abilities to form colonies on nutrient agar supplemented with nalidixic acid (NAL): (i) Nal s or wild type (no growth at 1 to 2 μg/ml); (ii) NalA r (growth at 40 μg/ml or higher); and (iii) NalB r (growth at 4 μg/ml, but no growth at 10 μg/ml). The NalA r group has a spectrum of sensitivity ranging from 60 to over 100 μg/ml. All Hfr strains of the NalA r and NalB r groups transfer NAL resistance to recipient cells at genetic loci which are at 42.5 ± 0.5 and 51 ± 1 min, respectively, on the Taylor-Trotter map. Some members of the NalA r group also have the genetic locus for NalB r . The nalA s allele is completely dominant to nalA r in a partial diploid configuration. In haploids, nalA r - nalB r is phenotypically NalA r ; nalA r - nalB s is NalA r ; and nalA s - nalB r is NalB r . The map location of nalA and the easy differentiation between NalA r and NalA s allow this marker to be used as a counterselector in bacterial conjugation experiments.