Expression of Silent Genes: Possible Interaction between DNA Gyrase and RNA Polymerase
Numerous studies indicate that DNA gyrase is an essential component of the transcription process. Supercoiled DNA is more readily transcribed than relaxed DNA (1–4). Inhibitors of DNA gyrase inhibit the transcription of some operons in vivo (5–7) and in vitro (8–9). Transcription is also affected in a ts gyrase B mutant when grown under nonpermissive conditions (10). It was recently shown that mutants lacking topoisomerase I can grow normally only with secondary mutations that compensate for the absence of topoisomerase I activity. Several of these secondary mutations map in gyrA or gyrB. These observations suggested that DNA superhelicity is a result of a balance between topoisomerase I and gyrase activities (11–12). It was shown further that mutations in gyrA or gyrB can activate the cryptic bgl operon, however, the results obtained with various mutants do not indicate a clear cut correlation between the degree of superhelicity and gene expression (12). The differential effect of DNA supercoiling on gene expression was recently discussed by Smith (13).
KeywordsSilent Gene Secondary Mutation Suppressor Phenotype Nonpermissive Condition Gyrase Activity
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