Modulation of gene expression by drugs affecting deoxyribonucleic acid gyrase

Nalidixic acid (Nal), a drug which affects deoxyribonucleic acid gyrase activity, inhibits the expression of catabolite-sensitive genes: the three maltose operons, the lactose and galactose operons, and the tryptophanase gene. A correlation between the degree of sensitivity to Nal and that to catabolite repression has been observed. The expression of the threonine and tryptophan operons, insensitive to catabolite repression, is insensitive to Nal. The expression of the lacZ gene under the control of the IQ promoter is activated by Nal. Strains carrying a mutation in the nalA locus are resistant to these effects. Novobiocin, which inhibits the negative supercoiling activity of deoxyribonucleic acid gyrase, affects expression of the operons similarly to Nal. The involvement of promoters in Nal and novobiocin action, as well as a possible role of in vivo negative supercoiling in the selectivity of gene expression, are discussed.