2018 Volume 6 Pages 105-118
LysR-type transcriptional regulators (LTTRs) comprise one of the largest families of transcriptional regulators in bacteria and control gene expression of various types of metabolic, virulence and physiological functions. LTTRs typically form homotetramers and require an inducer molecule(s) to activate the transcription of target genes. The N-terminal region of LTTRs contains a DNA-binding domain (DBD) with the winged helix-turn-helix motif that specifically binds the promoter region of target genes. The C-terminal region of LTTRs is connected to the DBD by a linker helix and forms the regulatory domain (RD) that contains a binding pocket for inducer molecules. Crystal structures of several LTTR family members together with their biochemical analyses have provided a potential mechanism for the initial process of transcriptional activation by LTTRs. First, helix α3 of the winged helix-turn-helix motif in DBD is supposed to distinguish the recognition binding site (RBS) in the promoter region, resulting in complex formation through interactions between two DBDs in the tetrameric LTTR and RBS. Formation of this complex seems to enable interactions between the other two DBDs in the LTTR tetramer and the activation binding site (ABS) in the promoter region. The binding of the tetrameric LTTR to both the RBS and ABS causes the promoter DNA to adopt a bent structure because the four DBDs in the tetrameric LTTR are arranged in a V-shaped manner at the bottom of the LTTR. Interaction of an inducer molecule(s) with the RD seems to cause a quaternary structural change of the LTTR that relaxes the bending angle of the promoter DNA with a concomitant shift of the bound DBDs at the ABS. These events facilitate recruitment of RNA polymerase to its binding site in the promoter region, which overlaps with the ABS for LTTR.