Independent regulation of H-NS-mediated silencing of the bgl operon at two levels: upstream by BglJ and LeuO and downstream by DnaKJ

Abstract

Silencing of theEscherichia coli bgloperon by the histone-like nucleoid-structuring protein H-NS occurs at two levels. Binding of H-NS upstream of the promoter represses transcription initiation, whilst binding within the coding region is also proposed to repress transcription elongation. The latter, downstream level of repression is counteracted by the protease Lon and, thus, silencing of thebgloperon is more effective inlonmutants. Transposon-mutagenesis screens for suppression of thislonphenotype onbglwere performed and insertion mutations disruptingrpoSandcrlwere obtained, as well as mutations mapping upstream of the open reading frames ofbglJ,leuOanddnaK. InrpoSandcrlmutants,bglpromoter activity is known to be higher. Likewise, as shown here,bglpromoter activity is increased in thebglJandleuOmutants, which express BglJ and LeuO constitutively. However, BglJ and LeuO have no impact on downstream repression. AdnaKJmutant was isolated for the first time in the context of thebgloperon. The mutant expresses lower levels of DnaK than the wild-type. Interestingly, in thisdnaKJ : : miniTn10mutant, downstream repression ofbglby H-NS is less effective, whilst upstream repression by H-NS remains unaffected. Together, the data show that the two levels ofbglsilencing by H-NS are regulated independently.