Structures of the transcriptional regulator BgaR, a lactose sensor.

Janet Newman,Thomas S Peat,Tom Nebl,Karine Caron

doi:10.1107/s2059798319008131

Abstract

The structure of BgaR, a transcriptional regulator of the lactose operon in Clostridium perfringens, has been solved by SAD phasing using a mercury derivative. BgaR is an exquisite sensor of lactose, with a binding affinity in the low-micromolar range. This sensor and regulator has been captured bound to lactose and to lactulose as well as in a nominal apo form, and was compared with AraC, another saccharide-binding transcriptional regulator. It is shown that the saccharides bind in the N-terminal region of a jelly-roll fold, but that part of the saccharide is exposed to bulk solvent. This differs from the classical AraC saccharide-binding site, which is mostly sequestered from the bulk solvent. The structures of BgaR bound to lactose and to lactulose highlight how specific and nonspecific interactions lead to a higher binding affinity of BgaR for lactose compared with lactulose. Moreover, solving multiple structures of BgaR in different space groups, both bound to saccharides and unbound, verified that the dimer interface along a C-terminal helix is similar to the dimer interface observed in AraC.

Highlights

Living organisms have evolved a broad range of sensing systems for the detection of small molecules, such as metabolites and nutrients, and of temperature and pH variations
This paper describes the purification, crystallization and structure determination by SAD phasing of the regulatory domain of BgaR bound to lactose and to lactulose, as well as a nominal apo form of the protein, in several different space groups
The shorter regulatory domain construct (BgaR1–170-thrombin-His6) only gave crystals when treated with a protease, which resulted in the removal of the C-terminal tag and five or six residues from the C-terminus

Summary

Introduction

Living organisms have evolved a broad range of sensing systems for the detection of small molecules, such as metabolites and nutrients, and of temperature and pH variations. Transcriptional regulators (TRs) play a major role in sensing small molecules and are a broad family of proteins that control cell development, cell differentiation and cell growth through the regulation of gene expression, and these proteins are found throughout all biological kingdoms. A small molecule such as a metabolite or a nutrient binds to the effector-binding domain, leading to a conformation change that alters the interaction of the DNA-binding domain with the target DNA, affecting the efficiency of gene transcription. TRs have been divided into a broad range of families based on structural and binding similarities. The structural divergence amongst the TRs of the GntR superfamily gives rise to six subfamilies: four main subfamilies (FadR, HutC, MocR and YtrA) and two minor subfamilies (AraC and PlmA) (Jain, 2015; Rigali et al, 2002)

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Conclusion