BglR protein, which belongs to the BglG family of transcriptional antiterminators, is involved in beta-glucoside utilization in Lactococcus lactis

Abstract

The bgl operon of Escherichia coli is involved in the utilization of P-glucoside sugars (25,36). This operon is cryptic in wild-type strains but can be rendered functional by a variety of spontaneous mutations (30,31,35). When functional, this operon is inducible by 3-glucosides. This control is exerted through transcription antitermination mediated by the BglG protein (3,22,25,33,34). Transcription initiates constitutively at the bgl promoter, but in the absence of 3-glucosides, most transcripts terminate at a p-independent terminator located in the leader region, immediately upstream of the bglG gene. In the presence of P-glucosides, the BglG protein binds to a specific sequence of the mRNA, overlapping part of the transcription terminator, thus preventing RNA polymerase from terminating. The binding of BglG is modulated by its phosphorylation by EIIBgi, an enzyme of the phosphoenolpyruvate:p-glucoside phosphotransferase system, which is involved in uptake and phosphorylation of P-glucosides (36). In the absence of 0-glucosides, P-EIIBgl phosphorylates BglG, which thus becomes unable to bind to RNA and act as an antiterminator (3, 4). If 3-glucosides are present, they are phosphorylated by P-EII'9' and P-BglG is dephosphorylated. The protein can thus bind to RNA, which results in transcription antitermination.Based on a strong conservation of the amino acid sequences of the regulatory proteins and of their putative RNA systems. These systems include the control of the Bacillus subtilis levansucrase sacB gene by SacY (7, 13), of the B. subtilis sacPA operon by SacT (6, 14, 39), of B. subtilis 13-glucan utilization by licT (40), of Lactobacillus casei lactose utilization (1), and of the Erwinia chrysanthemi P-glucoside phosphotransferase gene arbF by ArbG (15).In this article, we describe the Lactococcus lactis bglR gene, whose product shares homology with the BglG family of antiterminators. bglR is preceded by a transcription terminator partially overlapped by the conserved target sequence of the BglG antiterminators. Constitutive expression of bglR in E. coli increased the expression of a bglG::lacZ transcriptional fusion. The fact that the expression of BglG is autoregulated in E. coli suggests that BglG and BglR are functionally related. In L.lactis, the expression of bglR is positively controlled by P-glucosides and its disruption results in a deficiency in P-glucoside utilization. Taken together, these results indicate that BglR is a lactococcal counterpart of the E. coli BglG regulator of 3-glucoside utilization. MATERIALS AND METHODSBacterial strains and media. L. lactis subsp. lactis IL1403 (11) and derivatives, E. coli TG1 (18) and MA152 (24), and B. subtilis MT119 (41) were grown as previously described (8). Growth on various carbohydrates in a chemically defined broth (28,29,38), supplemented with the appropriate sugar at 1% final concentration, was monitored. The medium was inoculated with 1% of a culture grown overnight and then washed twice.DNA manipulations. Plasmids and chromosomal DNAs were e...

The LicT protein acts as both a positive and a negative regulator of loci within the bgl regulon of Streptococcus mutans

“…Antiterminator proteins are believed to be involved in the transcriptional regulation of b-glucoside-specific genes from Escherichia coli (Hall & Xu, 1992), Erwinia chrysanthemi (el Hassouni et al, 1990), Lactococcus lactis (Bardowski et al, 1994), Lactobacillus plantarum (Marasco et al, 2000), B. subtilis (Le Coq et al, 1995;Tobisch et al, 1997) and C. longisporum (Brown & Thomson, 1998). It is believed that these antiterminator proteins would bind to a ribonucleic antiterminator (RAT) site present in a specific mRNA secondary structure and would prevent the formation of a hairpin terminator structure that terminates transcription (Rutberg, 1997).…”

Antitermination of transcription of catabolic operons

“…The Gram-positive, low-GC bacterium Lactococcus lactis also appears to contain a bgl operon homologous to that of E. coli, although it has only been partially characterized (Bardowski et al, 1994).…”

Inducible gene expression and environmentally regulated genes in lactic acid bacteria

“…Transcription antitermination of E. coli BglG synthesis is [44] exerted by binding of the protein to a conserved RNA sequence partially overlapping the transcription terminator (Houman et al, 1990). This configuration, a transcription terminator and a 5' overlapping sequence with high similarity to the RNA binding site of these systems was present upstream of bglR (Bardowski et al,, 1994). bglR is positively autoregulated: in a strain carrying a bglR::lacZ fusion on the chromosome, constitutive overexpression of bglR from a plasmid resulted in 2-to 3-fold increased LacZ levels in the presence of/3-glucoside sugars.…”

“…A gene immediately downstream of the trp operon, bglR (see Figure 5) encodes a functional regulator of the BglG family (Bardowski et al, 1994). Proteins in this family positively control utilization of different sugars.…”