Chapter 6 : Trigger Enzymes: Coordination of Metabolism and Virulence Gene Expression

The localization of the PutA protein within the cell determines its role in proline catabolism. In the presence of exogenous proline, the trifunctional PutA enzyme catalyzes the two-step conversion of proline to glutamate, which may serve as a carbon and nitrogen source. This catabolically active, reduced form of PutA (PutAred) localizes to the membrane. The put divergon, encoding the proline transporter PutP and the PutA trigger enzyme, respectively, is expressed in the presence of proline. In the absence of proline, the oxidized PutA protein (PutAox) binds to the intergenic region of the putA and putP genes to repress their transcription. P5C, Δ1-pyrroline-5-carboxylate.

The β-glucoside permease controls the activity of the transcription antiterminator protein BglG in response to β-glucoside availability. In the presence of β-glucosides, the sugar is taken up by the β-glucoside permease of the PTS and concomitantly phosphorylated. The phosphoryl group is derived from phosphoenolpyruvate (PEP) and transferred via the phosphocarriers Enzyme I (EI) and HPr to the EIIB component of the β-glucoside permease. Under these conditions, the transcription-antiterminator protein BglG binds a stem-loop structure of the bgl mRNA, thereby preventing the formation of a terminator structure, and the transcription of the bgl mRNA can continue. Inactivation of the antiterminator protein BglG occurs in the absence of β-glucosides. BglG receives a phosphoryl goup from the β-glucoside permease and is now unable to bind the bgl mRNA. The formation of a termination structure occurs and the transcription of the bgl operon is aborted. Pyr, pyruvate.

Control of CymR DNA-binding activity by CysK. In the presence of cysteine, the acetyltransferase CysE is inhibited and the O-acetyl-serine (OAS)-thiol-lyase CysK forms a complex with the transcription factor CymR. The protein complex binds to the CymR-regulated genes and prevents transcription. At low cysteine levels, the OAS-thiol-lyase converts serine and acetyl-CoA to OAS, which serves as the substrate for CysK to produce cysteine.

Evolutionary stages of (trigger) enzymes A. Conventional enzymes (E), such as the β-galactosidase LacZ, catalyze metabolic reactions without controlling gene expression through modulating the activity of a transcription factor (TF). B. Bifunctional trigger enzymes (TEs) such as the glutamate dehydrogenase (GDH) from B. subtilis can control the activity of TFs by a direct protein-protein interaction. It has been suggested that the metabolites that are converted by the GDH also directly modulate the activity of the GDH-controlled TF, GltC. C. TEs like the glutamine synthetase (GS) from B. subtilis control the activities of TFs that do not response to metabolites. D. TFs such as the trifunctional PutA enzyme may have acquired a DNA-binding motif, which allows the enzyme to regulate gene expression depending of the metabolic state of the cell. E. TFs like BzdR from B. japonicum may be composed of a DNA-binding domain and an enzymatic domain that has lost its catalytic activity during evolution. These TE sense metabolites without converting them. S, substrate; P, product.