Motile bacteria respond to environmental cues to move to more favorable locations. The components of the chemotaxis signal transduction systems that mediate these responses are highly conserved among prokaryotes including both eubacterial and archael species. The best‐studied system is that found in Escherichia coli. Attractant and repellant chemicals are sensed through their interactions with transmembrane chemoreceptor proteins that are localized in multimeric assemblies at one or both cell poles together with a histidine protein kinase, CheA, an SH3‐like adaptor protein, CheW, and a phosphoprotein phosphatase, CheZ. These multimeric protein assemblies act to control the level of phosphorylation of a response regulator, CheY, which dictates flagellar motion. Bacterial chemotaxis is one of the most‐understood signal transduction systems, and many biochemical and structural details of this system have been elucidated. This is an exciting field of study because the depth of knowledge now allows the detailed molecular mechanisms of transmembrane signaling and signal processing to be investigated. BioEssays 28:9–22, 2006. © 2005 Wiley Periodicals, Inc.

Structure-function studies reveal that Rap proteins have distinct, nonoverlapping surfaces that interact with different cellular targets, and that for antiactivator RapF, one surface mimics DNA to bind a response regulator DNA binding domain, thereby sterically preventing the activity of this transcription transactivator.

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Escherichia coli chemotaxis is mediated by membrane receptor͞ histidine kinase signaling complexes. Fusing the cytoplasmic domain of the aspartate receptor, Tar, to a leucine zipper dimerization domain produces a hybrid, lzTar C, that forms soluble complexes with CheA and CheW. The three-dimensional reconstruction of these complexes was different from that anticipated based solely on structures of the isolated components. We found that analogous complexes self-assembled with a monomeric cytoplasmic domain fragment of the serine receptor without the leucine zipper dimerization domain. These complexes have essentially the same size, composition, and architecture as those formed from lzTar C. Thus, the organization of these receptor͞signaling complexes is determined by conserved interactions between the constituent chemotaxis proteins and may represent the active form in vivo. To understand this structure in its cellular context, we propose a model involving parallel membrane segments in receptor-mediated CheA activation in vivo.CheA ͉ histidine kinase ͉ serine receptor ͉ signal transduction

Under conditions of nutrient limitation and high population density, the bacterium Bacillus subtilis can initiate a variety of developmental pathways. The signaling systems regulating B. subtilis differentiation are tightly controlled by switch proteins called Raps, named after the founding members of the family, which were shown to be response regulator aspartate phosphatases. A phr gene encoding a secreted pentapeptide that regulates the activity of its associated Rap protein was previously identified downstream of 8 of the chromosomally encoded rap genes. We identify and validate here the sequence of an atypical Phr peptide, PhrH, by in vivo and in vitro analyses. Using a luciferase reporter bioassay combined with in vitro experiments, we found that PhrH is a hexapeptide (TDRNTT), in contrast to the other characterized Phr pentapeptides. We also determined that phrH expression is driven by a promoter lying within rapH. Unlike the previously identified dedicated H -driven phr promoters, it appears that phrH expression most likely requires A . Furthermore, we show that PhrH can antagonize both of the known activities of RapH: the dephosphorylation of Spo0F and the sequestration of ComA, thus promoting the development of spores and the competent state. Finally, we propose that PhrH is the prototype of a newly identified class of Phr signaling molecules consisting of six amino acids. This class likely includes PhrI, which regulates RapI and the expression, excision, and transfer of the mobile genetic element ICEBs1.