his operons of Escherichia coli and Salmonella typhimurium are regulated by DNA supercoiling.

The hisW mutations of Salmonella typhimurium are highly pleiotropic mutations that elevate his operon expression, reduce ilv gene expression, alter stable RNA metabolism, and confer defective growth properties. The hisW mutations are highly linked to a naladixic acid-resistant gyrA mutation of S. typhimurium. Multicopy recombinant plasmids containing the Escherichia coli gyrA gene are able to complement both the growth defects and the elevated his operon expression associated with the hisW mutations. We conclude that hisW mutations are alleles of the gyrA gene. The hisU1820 mutant of S. typhimurium exhibits many of the same phenotypes as hisW mutants. Several lines of evidence, including high transduction linkage to recF, suggest that hisU1820 is an allele of gyrB. Finally, well-characterized gyrA and gyrB alleles of E. coli are also his regulatory mutations. We propose that a wild-type degree of chromosomal superhelicity is required for maximal production of histidyl-tRNA and normal his operon regulation.