In this study, we re‐investigated the previously characterized RcsC (sensor His‐kinase) → RcsB (response regulator) phosphorelay system that is involved in the regulation of capsular polysaccharide synthesis in Escherichia coli. The previously proposed model hypothesized the occurrence of a direct phosphotransfer from RcsC to RcsB in response to an unknown external stimulus. As judged from the current general view as to the His → Asp phosphorelay, this RcsC → RcsB framework is somewhat puzzling, because RcsC appears to contain both a His‐kinase domain and a receiver domain, but not a histidine (His)‐containing phosphotransmitter domain (e.g. HPt domain). We thus suspected that an as yet unknown mechanism might be underlying in this particular His → Asp phosphorelay system. Here, we provide several lines of in vivo and in vitro evidence that a novel and unique His‐containing phosphotransmitter (named YojN) is essential for this signalling system. A revised model is proposed in which the multistep RcsC → YojN → RcsB phosphorelay is implicated. It was also demonstrated that this complex signalling system is somehow involved in the modulation of a characteristic behaviour of E. coli cells during colony formation on the surface of agar plates, namely swarming.

Background: Widespread bacterial signal transduction circuits are generally referred to as`twocomponent systems' or`histidine (His)-to-aspartate (Asp) phosphorelays.' In Escherichia coli, as many as 30 distinct His-to-Asp phosphorelay signalling pathways operate in response to a wide variety of environmental stimuli, such as medium osmolarity and anaerobiosis. In this regard, it is of interest whether or not some of them together constitute a network of signalling pathways through a physiologically relevant mechanism (often referred to as cross-regulation'). We have addressed this issue, with special reference to the osmo-responsive EnvZ and anaero-responsive ArcB phosphorelay signalling pathways in E. coli.

No abstract

The Escherichia coli sdhCDAB operon encodes succinate dehydrogenase, an enzyme complex involved in the tricarboxylic acid (TCA) cycle. Expression of this operon is under complex transcriptional regulation in response to growth conditions, such as anaerobiosis and carbon sources. Typically, the expression of sdhCDAB is known to be subjected to "an aerobic repression" and "a glucose repression." The molecular mechanism underlying the anaerobic repression has been well documented, involving both the ArcB-ArcA two-component system and the Fnr global anaerobic regulator. However, the mechanism underlying the glucose repression is not yet clear, because the involvement of the general catabolite regulators such as CRP and CRA has been dismissed. In this study, we conducted a series of genetic analyses to identify the regulator gene(s) involved in the glucose repression of sdh. The results demonstrate that the EIICB(Glc) protein (the ptsG gene product), a component of the major glucose transporter, acts as a crucial mediator in glucose repression. These results support the view that the EIICB(Glc) protein functions not only as a glucose transporter, but also as a glucose-sensing signal transducer that modulates the glucose repression of the sdhCDAB operon.