Effect of nalidixic acid and novobiocin on pBR322 genetic expression in Escherichia coli minicells

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Abstract
The effects of 2 DNA gyrase inhibitors, nalidixic acid and novobiocin, on the expression of plasmid pBR322 in E. coli minicells were studied. Quantitative estimates of the synthesis of pBR322-coded polypeptides in novobiocin-treated minicells showed that the synthesis of a polypeptide of MW 34,000 (the tetracycline resistance protein) was reduced to 11-20% of control levels, whereas the amount of a polypeptide of 30,500 (the .beta.-lactamase precursor) was increased to as much as 200%. Nalidixic acid affected the synthesis of the tetracycline resistance protein similarly to novobiocin, although to a lesser extent. The effects of nalidixic acid were not observed in a nalidixic-resistant mutant; those induced by novobiocin were only partially suppressed in a novobiocin-resistant mutant. The synthesis of one of the inducible tetracycline-resistant proteins (MW 34,000) coded by plasmid pSC101 was also reduced in nalidixic acid- and novobiocin-treated minicells. The gyrase inhibitors apparently modified the interaction of RNA polymerase with some promoters, either by decreasing the supercoiling density of plasmid DNA or by altering the association constant of the gyrase to specific DNA sites.