The BglG group of antiterminators: a growing family of bacterial regulators

The Escherichia coli antiterminator protein BglG stabilizes the 5'region of the bgl mRNA

The beta-glucoside utilization (bgl) genes of Escherichia coli are positively regulated by the product of the bglG gene, which functions as an antiterminator by binding to specific sequences present within the bgl mRNA. BglG is inactivated by phosphorylation in the absence of beta-glucosides by BglF, the bgl-specific component of the phosphotransferase system (PTS). Here, we present evidence for an additional function for BglG, namely the stabilization of the 5' end of the bgl mRNA. Half-life measurements of the promoter-proximal region of the bgl mRNA indicate a five fold enhancement of stability in the presence of active (unphosphorylated) BglG. This enhancement is lost when the binding of BglG to mRNA is prevented by deletion of the binding site. Interestingly, stabilization by BglG does not extend to downstream sequences. The enhanced stability of the upstream sequences suggest that BglG remains bound to its target on the mRNA even after the downstream sequences have been degraded. Implications of these observations for the mechanism of positive regulation of the operon by BglG are discussed.

Analysis of the beta-glucoside utilization (bgl) genes of Shigella sonnei: evolutionary implications for their maintenance in a cryptic state

The pattern of expression of the genes involved in the utilization of aryl beta-glucosides such as arbutin and salicin is different in the genus Shigella compared to Escherichia coli. The results presented here indicate that the homologue of the cryptic bgl operon of E. coli is conserved in Shigella sonnei and is the primary system involved in beta-glucoside utilization in the organism. The organization of the bgl genes in S. sonnei is similar to that of E. coli; however there are three major differences in terms of their pattern of expression. (i) The bglB gene, encoding phospho-beta-glucosidase B, is insertionally inactivated in S. sonnei. As a result, mutational activation of the silent bgl promoter confers an Arbutin-positive (Arb(+)) phenotype to the cells in a single step; however, acquiring a Salicin-positive (Sal(+)) phenotype requires the reversion or suppression of the bglB mutation in addition. (ii) Unlike in E. coli, a majority of the activating mutations (conferring the Arb(+) phenotype) map within the unlinked hns locus, whereas activation of the E. coli bgl operon under the same conditions is predominantly due to insertions within the bglR locus. (iii) Although the bgl promoter is silent in the wild-type strain of S. sonnei (as in the case of E. coli), transcriptional and functional analyses indicated a higher basal level of transcription of the downstream genes. This was correlated with a 1 bp deletion within the putative Rho-independent terminator present in the leader sequence preceding the homologue of the bglG gene. The possible evolutionary implications of these differences for the maintenance of the genes in the cryptic state are discussed.