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

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 mug/ml); (ii) NalA(r) (growth at 40 mug/ml or higher); and (iii) NalB(r) (growth at 4 mug/ml, but no growth at 10 mug/ml). The NalA(r) group has a spectrum of sensitivity ranging from 60 to over 100 mug/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.