Crystal Structure of the Full-Length Sorbitol Operon Regulator SorC from Klebsiella pneumoniae: Structural Evidence for a Novel Transcriptional Regulation Mechanism

SorC transcriptional regulators are common regulators in prokaryotes. Here we report the first crystal structure of a full-length SorC, the sorbitol operon regulator SorC from Klebsiella pneumoniae, the prototype of its family. SorC was found to be a homotetramer (which seems to be the biologically active form) that is able to recognize its DNA operator. The tetramer can be regarded as a dimer of dimers, with each dimer being composed of two identical subunits in different conformations. The DNA-binding domains divergently protrude from the core of the tetramer, suggesting that SorC may bind its operator in two distinct regions. The sugar-binding domain presents the same fold identified in members of the SorC family that shows some features identified as specific for sugar recognition. An in silico analysis shows that the localization of the putative sugar-binding site is close to the dimeric interfaces. This supports the proposal of a new mechanism of transcriptional regulation that is in complete agreement with functional studies recently reported on a protein belonging to the same family.