CheA protein, a central regulator of bacterial chemotaxis, belongs to a family of proteins that control gene expression in response to changing environmental conditions

Regulation of the phosphate regulon of Escherichia coli: analysis of mutant phoB and phoR genes causing different phenotypes

The phoB gene product of Escherichia coli is the transcriptional activator for the genes in the phosphate regulon as well as for phoB itself, all of which are induced by phosphate starvation. The phoR gene product modulates PhoB function in response to the phosphate concentrations in the medium. We quantitatively compared the levels of expression of the phoA, phoB, phoE, and pstS genes in several phoB mutants with different phenotypes by constructing operon fusions of these genes with the gene for chloramphenicol acetyltransferase. Although all the phoB mutants examined had little activator function for phoA, three among the four mutants showed various levels of the activator function for phoB, pstS, and phoE. To study the functional motifs of the PhoB and PhoR proteins, we cloned and sequenced the four classical phoB and six phoR mutant genes. All of the phoB mutations and one of the phoR mutations were missense mutations, and most of the altered amino acids were in the highly conserved amino acids among the regulatory proteins homologous to PhoB or PhoR protein, such as the OmpR, SfrA, and VirG proteins or the EnvZ, CpxA, and VirA proteins. The other five phoR mutations were nonsense mutations.

Activation of bacterial porin gene expression by a chimeric signal transducer in response to aspartate

The Tar chemoreceptor of Escherichia coli is a membrane-bound sensory protein that facilitates bacterial chemotaxis in response to aspartate. The EnvZ molecule has a membrane topology similar to Tar and is a putative osmosensor that is required for osmoregulation of the genes for the major outer membrane porin proteins, OmpF and OmpC. The cytoplasmic signaling domain of Tar was replaced with the carboxyl portion of EnvZ, and the resulting chimeric receptor activated transcription of the ompC gene in response to aspartate. The activation of ompC by the chimeric receptor was absolutely dependent on OmpR, a transcriptional activator for ompF and ompC.

Characterization of the virA virulence gene of the nopaline plasmid, pTiC58, of Agrobacterium tumefaciens

We have determined the complete nucleotide sequence of a 4.8 kilobase fragment encompassing the virA locus of the nopaline-type plasmid, pTiC58, of Agrobacterium tumefaciens. virA is composed of a single open reading frame of 2499 nucleotides, capable of encoding a protein of 91.3 kiloDaltons. A trpE::virA gene fusion was used to confirm the reading frame of virA. High nucleotide and amino acid sequence homologies were observed between pTiC58 virA and the virA sequences of three octopine-type plasmids. Strong homologies in amino acid sequence were observed between pTiC58 VirA and seven bacterial proteins which control various regulons. Two hydrophobic domains within VirA are also consistent with a model in which VirA acts as a membrane-bound sensor of plant signal molecules.

Regulation of enterobacterial cephalosporinase production: the role of a membrane-bound sensory transducer

In clinical isolates of Enterobacter cloacae, resistance to the newer beta-lactam antibiotics often results from overproduction of a cephalosporinase encoded by the beta-lactam-inducible ampC gene. Regulation of ampC is controlled by the divergently expressed activator gene, ampR, and a second unlinked locus. In this presentation we show that although Escherichia coli has lost its ampR gene it has retained the second regulatory locus and that this comprises the bicistronic ampDE operon. Genetic and biochemical studies define the ampD gene as encoding a repressor for ampC transcription whereas the ampE gene product is a cytoplasmic membrane protein. Inactivation of the AmpD protein by mutation causes massive overproduction of cephalosporinase which, in E. cloacae, can terminate in therapeutic failure. In contrast, loss of AmpE results in a total block in induction, despite the presence of the activator, AmpR.

Phosphorylation of OmpR by the osmosensor EnvZ modulates expression of the ompF and ompC genes in Escherichia coli

EnvZ and OmpR, the regulatory proteins for ompF and ompC expression in Escherichia coli, belong to a modulator-effector family of regulatory proteins which are essential for the response to environmental signals. We show that the soluble cytoplasmic domain of the transmembrane modulator protein EnvZ is phosphorylated in vitro by [gamma-32P]-ATP. We also demonstrate that the phosphate group can, in turn, be transferred to the transcription activator protein OmpR. The pH stability properties of the phosphate groups linked to EnvZ indicate that this molecule contains histidyl phosphate. The invariant His-243 of EnvZ corresponds to the phosphorylated His-48 of the chemotactic modulator protein CheA. Substitution of His-243 with valine produces an EnvZ that is refractory to phosphorylation and can no longer catalyze the transfer of phosphate to OmpR. Furthermore, in a delta envZ strain of E. coli, containing the envZ Val-243 plasmid, ompC expression is elevated 7-fold relative to that found in cells carrying the wild-type envZ plasmid. Based on these results we propose a model in which the phosphorylated state of OmpR modulates the expression of the ompF and ompC genes.

Structure of genes narL and narX of the nar (nitrate reductase) locus in Escherichia coli K-12

narL and narX mediate nitrate induction of nitrate reductase synthesis and nitrate repression of fumarate reductase synthesis. We report here the nucleotide sequences of narL and narX. The deduced protein sequences aid in defining distinct subclasses of regulators and sensors in the family of two-component regulatory proteins.

Physiological and genetic responses of bacteria to osmotic stress

The capacity of organisms to respond to fluctuations in their osmotic environments is an important physiological process that determines their abilities to thrive in a variety of habitats. The primary response of bacteria to exposure to a high osmotic environment is the accumulation of certain solutes, K+, glutamate, trehalose, proline, and glycinebetaine, at concentrations that are proportional to the osmolarity of the medium. The supposed function of these solutes is to maintain the osmolarity of the cytoplasm at a value greater than the osmolarity of the medium and thus provide turgor pressure within the cells. Accumulation of these metabolites is accomplished by de novo synthesis or by uptake from the medium. Production of proteins that mediate accumulation or uptake of these metabolites is under osmotic control. This review is an account of the processes that mediate adaptation of bacteria to changes in their osmotic environment.

Coordinate regulation and sensory transduction in the control of bacterial virulence

Genes and operons that encode bacterial virulence factors are often subject to coordinate regulation. These regulatory systems are capable of responding to various environmental signals that may be encountered during the infectious cycle. For some pathogens, proteins that mediate sensory transduction and virulence control are similar to components of other bacterial information processing systems. Understanding the molecular mechanisms governing global regulation of pathogenicity is essential for understanding bacterial infectious diseases.

Phosphorylation of nitrogen regulator I (NRI) of Escherichia coli

It has previously been shown that phosphorylated nitrogen regulator I (NRI-phosphate) is the activator responsible for increasing the transcription of glnA, the structural gene for glutamine synthetase, and that NRII catalyzes the transfer of the gamma-phosphate of ATP to NRI. We have now shown that the reaction of ATP with NRII results in the reversible transfer of the gamma-phosphate of ATP to a histidine residue of NRII. In turn, NRII-phosphate transfers its phosphate reversibly to an aspartic residue of NRI. NRI-phosphate is hydrolyzed to NRI and inorganic phosphate in a divalent cation-requiring autocatalytic reaction.

Histidine phosphorylation and phosphoryl group transfer in bacterial chemotaxis

A cascade of protein phosphorylation, initiated by autophosphorylation of the CheA protein, may be important in the signal transduction pathway of bacterial chemotaxis. A proteolytic fragment of CheA cannot autophosphorylate, but can still transfer phosphate to proteins that generate excitation and adaptation signals. The site of CheA phosphorylation is His 48; mutants altered at this position are non-chemotactic. Similar mechanisms of transient protein phosphorylation and phosphoryl group transfer seem to be involved in processing sensory data and in activating specific gene expression.

Deduced polypeptides encoded by the Bacillus subtilis sacU locus share homology with two-component sensor-regulator systems

The sacU locus has been cloned by using two independent strategies, and the presence of two open reading frames was deduced from the nucleotide sequence. Open reading frame 1 encodes a 45,000-dalton polypeptide that is similar to the products of the Salmonella typhimurium cheA and Escherichia coli cpxA genes, which act as sensory transducers. Open reading frame 2 encodes a 26,000-dalton polypeptide that is similar to a family of transcriptional activators, including the products of the Bacillus subtilis spoOA and spoOF and the E. coli ompR and dye genes. These similarities suggest that the products of the B. subtilis sacU locus form a sensor-transducer couple, which functions to relay information about specific environmental changes to the transcription apparatus.

Comparison of the chemotactic behaviour of Rhizobium leguminosarum with and without the nodulation plasmid

The chemotactic behaviour of a strain of Rhizobium leguminosarum biovar viciae was investigated. The flavanoids apigenin and naringenin, inducers of transcription of the nodulation (nod) genes, were both potent attractants but hesperitin, another flavone nod gene inducer, was not. The response of strains containing the Sym plasmid pRL1Jl to apigenin and naringenin was significantly greater than the response of a strain cured of the plasmid, although both strains gave a positive response. Addition of the flavanol kaempferol, an antagonist of nod gene induction, had no detectable effect on the chemotactic response to naringenin or apigenin, but was itself found to be an attractant. The attractant response to a variety of amino acids and sugars was not affected by the presence of the Sym plasmid. Homoserine, the most abundant nitrogenous compound in legume exudates, was also found to be an attractant. However, although the Sym plasmid is required for the biovar to metabolize homoserine as a carbon source, it was not required for the chemotactic response. A group of membrane proteins showed increased methylation in response to stimulation with serine. There was no measurable change in methylation after stimulation with apigenin.

Acetyladenylate plays a role in controlling the direction of flagellar rotation

Cells of Escherichia coli deleted for genes that code for the transducers and all the known cytoplasmic Che proteins except CheY responded reversibly to the addition of acetate by spinning their flagellar motors clockwise. By varying growth conditions and using metabolic inhibitors and mutants deficient in acetate metabolism, this effect was shown to require acetate-CoA synthetase [acetate:CoA ligase (AMP-forming); EC 6.2.1.1], an enzyme that catalyzes the formation of acetyl-CoA from acetate by an acetyladenylate intermediate. A mutant deficient in this enzyme but retaining the chemotaxis genes was deficient for chemotaxis. Thus, acetyladenylate appears to play a role in generating clockwise rotation at the level of CheY or the motor.

Crosstalk between bacterial chemotaxis signal transduction proteins and regulators of transcription of the Ntr regulon: evidence that nitrogen assimilation and chemotaxis are controlled by a common phosphotransfer mechanism

We demonstrate by using purified bacterial components that the protein kinases that regulate chemotaxis and transcription of nitrogen-regulated genes, CheA and NRII, respectively, have cross-specificities: CheA can phosphorylate the Ntr transcription factor NRI and thereby activate transcription from the nitrogen-regulated glnA promoter, and NRII can phosphorylate CheY. In addition, we find that a high intracellular concentration of a highly active mutant form of NRII can suppress the smooth-swimming phenotype of a cheA mutant. These results argue strongly that sensory transduction in the Ntr and Che systems involves a common protein phosphotransfer mechanism.

Transmitter and receiver modules in bacterial signaling proteins

Prokaryotes are capable of sophisticated sensory behaviors. We have detected sequence motifs in bacterial signaling proteins that may act as transmitter or receiver modules in mediating protein-protein communication. These modules appear to retain their functional identities in many protein hosts, implying that they are structurally independent elements. We propose that the fundamental activity characterizing these domains is specific recognition and association of matched modules, accompanied by conformational changes in one or both of the interacting elements. Signal propagation is a natural consequence of this behavior. The versatility of this information-processing strategy is evident in the chemotaxis machinery of Escherichia coli, where proteins containing transmitters or receivers are linked in "dyadic relays" to form complex signaling networks.