Bgl Promoter Research Articles

Silencing of Escherichia coli bgl promoter by flanking sequence elements.

Silencing of a transcriptional unit by flanking sequence elements has so far only been described for eukaryotic systems. Here, a similar system is described in bacteria. The Escherichia coli bgl operon (beta-glucoside utilization) is normally cryptic due to very low promoter activity. However, low activity is not attributable to the quality of the promoter itself but is caused by its chromosomal context. The bgl promoter is perfectly active when tested outside of its normal context of a stretch of a few hundred base pairs. In addition, other promoters become inactivated when placed into the bgl region. Both the deletion of an upstream sequence element and the replacement of sequences located downstream result in promoter de-repression, demonstrating that silencing of promoters within this stretch of DNA in vivo is an active process brought about by the combined action of upstream and downstream chromosomal elements.

IS5: a mobile enhancer of transcription in Escherichia coli.

The cryptic bgl operon of Escherichia coli is activated by the spontaneous insertion of mobile DNA elements. Screening of a collection of such mutations revealed insertion of the 1195-base-pair element IS5 into various positions both upstream and downstream of the bgl promoter P0. Activation of the operon was in all cases attributable to enhancement of P0 activity. Introduction of internal deletions into IS5 almost completely abolished P0 enhancement, demonstrating that enhancement is not simply the result of mutational inactivation of some inhibitory sequences. Intact copies of IS5 in trans restored the enhancing activity of the deletion derivatives. The trans-activator is encoded by IS5 gene ins5A, an essential transposition function. Activation of gene expression by means of interaction of a defective mobile element in cis with functions encoded by a nondefective element in trans has so far been described only for a maize controlling element.

Regulation of the bgl operon of Escherichia coli by transcriptional antitermination.

The bgl operon of Escherichia coli encodes all functions necessary for the regulated uptake and utilization of aryl beta-glucosides. The operon is unusual, however, in that it is cryptic in wild-type strains, requiring activation by mutational events. The vast majority of these mutations are due to transposition of insertion elements into the promoter region of the operon. In this report we show that integration of IS5 into the vicinity of the bgl promoter (P0) enhances its activity by greater than 60-fold thereby activating the operon. In the activated state the operon is subject to induction by substrate. Recent studies have shown that induction of the bgl operon by substrate involves antitermination within the leader of the operon. We now show that substrate-dependent regulation involves specific termination/antitermination of transcription at two signal structures flanking the first gene of the operon, bglG. Antitermination is mediated by the product of gene bglG. In the absence of substrate this antitermination is prevented by the action of the product of gene bglF (the second gene of the operon), which encodes the beta-glucoside-specific transport protein (enzymeIIBgl of the phosphoenolpyruvate-dependent phosphotransferase system, PTS) resulting in repression of the operon. The bgl promoter (P0) is not subject to substrate-dependent regulation. The bgl operon has two additional promoters (P1 and P2) located within the terminators, which could also participate in regulation.

A bacterial gene involved in transcription antitermination: Regulation at a rho-independent terminator in the bgl operon of E. coli

We have investigated the mechanism of regulation of the bgl operon in Escherichia coli K-12. A regulatory region has been located downstream of the bgl promoter, revealing a 130 base leader containing a sequence from +64 to +112 characteristic of a rho-independent terminator. In vitro, over 90% of the transcripts initiated at the bgl promoter terminate within the leader, at the 3′ end of the terminator. Transcriptional fusions containing the terminator require the bglC gene in trans for expression; fusions deleted for the sequence show bglC-independent expression. A mutation resulting in partially constitutive expression of the fusion maps within the terminator. We propose that the bglC gene product mediates positive regulation of the bgl operon by functioning as an antiterminator at the rho-independent terminator located within the leader.