Escherichia coli gyr A 43 and gyr B 203 alleles conferring temperature-sensitive (ts) growth encoded Gly751 → Asp and Pro171 → ser substitutions in the DNA gyrase A and B subunits, respectively. A plasmid-borne gyr A 43 allele wasg genetically dominant over a chromosomal quinolone-resistant gyr A gene at 30°C but not at 42°C. These results and others confirm the ts phenotype of the mutation, the first to be identified in the C-terminal DNA binding/complex stabilizing domain of gyrase A protein. By contrast, the Pro171 → Ser mutation is located near the ATP-binding site of gyrase B protein and could interfere with energy coupling during DNA supercooling. These data are discussed in regard to recently described gyr A (ts) mutations that affect the control of chromosome segregation.
