Bacillus subtilis GabR, a Protein with DNA-binding and Aminotransferase Domains, is a PLP-dependent Transcriptional Regulator

Bacillus subtilis GabR is a member of a poorly characterized but widespread family of chimeric bacterial proteins that have apparent DNA binding and aminotransferase domains. GabR positively regulates expression of the gabTD operon responsible for utilization of γ-aminobutyric acid (GABA) and represses the divergently transcribed gabR gene. Purified GabR bound specifically to the DNA region overlapping the −35 region of the gabT promoter and the −10 and +1 regions of the gabR promoter. Two 6 bp direct repeats located at the ends of this region appeared to be essential for GabR binding. In transcription reactions in vitro, GabR alone repressed expression from the gabR promoter but activated expression from the gabT promoter only in the presence of GABA and pyridoxal 5′-phosphate, an essential cofactor of aminotransferases. A similar requirement for pyridoxal 5′-phosphate and GABA for GabR-mediated transcription activation was shown in vivo. In vitro this requirement could be partially satisfied with pyridoxamine 5′-phosphate and succinic semialdehyde, the products of a GABA-dependent aminotransferase half-reaction. We hypothesize that the GabR-catalyzed aminotransferase-like reaction between GABA and pyridoxal 5′-phosphate is essential for GabR action as a transcriptional activator.