Characterization of the dimerization domain in BglG, an RNA-binding transcriptional antiterminator from Escherichia coli

The Escherichia coli transcriptional antiterminator protein BglG inhibits t ranscription termination of the bgl operon in response to the presence of b eta-glucosides in the growth medium. BglG is an RNA-binding protein that re cognizes a specific sequence partially overlapping the two terminators with in the bgl transcript. The activity of BglG is determined by its dimeric st ate which is modulated by reversible phosphorylation. Thus, only the nonpho sphorylated dimer binds to the RNA target site and allows readthrough of tr anscription. Genetic systems which test dimerization and antitermination in vivo were used to map and delimit the region which mediates BglG dimerizat ion. We show that the last 104 residues of BglG are required for dimerizati on. Any attempt to shorten this region from the ends or to introduce intern al deletions abolished the dimerization capacity of this region. A putative leucine zipper motif is located at the N terminus of this region. The role of the canonical leucines in dimerization was demonstrated by their substi tution. Our results also suggest that the carboxy-terminal 70 residues, whi ch follow the leucine zipper, contain another dimerization domain which doe s not resemble any known dimerization motif. Each of these two regions is n ecessary but not sufficient for dimerization. The BglG phosphorylation site , His(208), resides at the junction of the two putative dimerization domain s. Possible mechanisms by which the phosphorylation of BglG controls its di merization and thus its activity are discussed.