Phosphorylation/dephosphorylation of the receiver module at the conserved aspartate residue controls transphosphorylation activity of histidine kinase in sensor protein ArcB of Escherichia coli

Involvement of Fnr and ArcA in anaerobic expression of the tdc operon of Escherichia coli

Anaerobic expression of the tdcABC operon in Escherichia coli, as measured by LacZ activity from single-copy tdc-lacZ transcriptional and translational fusions, is greatly reduced in strains lacking two global transcriptional regulators, Fnr and ArcA. The nucleotide sequence of the tdc promoter around -145 shows significant similarity with the consensus Fnr-binding site; however, extensive base substitutions within this region had no effect on Fnr regulation of the tdc genes. A genetic analysis revealed that the effect of Fnr on tdc is not mediated via ArcA. Furthermore, addition of cyclic AMP to the anaerobic incubation medium completely restored tdc expression in fnr and arcA mutants as well as in strains harboring mutations in the Fnr- and ArcA-dependent pfl gene and the Fnr-regulated glpA and frd genes. These results, taken together with the earlier finding that tdc expression is subject to catabolite repression by intermediary metabolites, strongly suggest that the negative regulatory effects of mutations in the fnr and arcA genes are mediated physiologically due to accumulation of a metabolite(s) which prevents tdc transcription in vivo.

Activated alleles of yeast SLN1 increase Mcm1-dependent reporter gene expression and diminish signaling through the Hog1 osmosensing pathway

Two-component signal transduction systems involving histidine autophosphorylation and phosphotransfer to an aspartate residue on a receiver molecule have only recently been discovered in eukaryotes, although they are well studied in prokaryotes. The Sln1 protein of Saccharomyces cerevisiae is a two-component regulator involved in osmotolerance. Phosphorylation of Sln1p leads to inhibition of the Hog1 mitogen-activated protein kinase osmosensing pathway. We have discovered a second function of Sln1p by identifying recessive activated alleles (designated nrp2) that regulate the essential transcription factor Mcm1. nrp2 alleles cause a 5-fold increase in the activity of an Mcm1-dependent reporter, whereas deletion of SLN1 causes a 10-fold decrease in reporter activity and a corresponding decrease in expression of Mcm1-dependent genes. In addition to activating Mcm1p, nrp2 mutants exhibit reduced phosphorylation of Hog1p and increased osmosensitivity suggesting that nrp2 mutations shift the Sln1p equilibrium toward the phosphorylated state. Two nrp2 mutations map to conserved residues in the receiver domain (P1148S and P1196L) and correspond to residues implicated in bacterial receivers to control receiver phosphorylation state. Thus, it appears that increased Sln1p phosphorylation both stimulates Mcm1p activity and diminishes signaling through the Hog1 osmosensing pathway.

Transphosphorylation of the TorR response regulator requires the three phosphorylation sites of the TorS unorthodox sensor in Escherichia coli

Two-component regulatory systems allow cells to adapt to environmental changes. In Escherichia coli, the TorS/TorR two-component system induces the expression of the tor structural operon encoding the trimethylamine N-oxide reductase respiratory system in response to substrate availability. TorS belongs to a sensor subfamily that includes a classical transmitter domain, a receiver, and a C-terminal alternative transmitter domain. The histidine phosphorylation sites of each TorS transmitter domain and the aspartate phosphorylation site of the TorS receiver were individually changed by site-directed mutagenesis. All three phosphorylation sites proved essential for in vivo induction of the tor structural operon and for in vitro transphosphorylation of the cognate TorR response regulator. The His to Gln change in the classical transmitter domain abolished TorS autophosphorylation, whereas TorS underwent significant autophosphorylation when the phosphorylation site of its receiver or alternative transmitter was changed. Complementation between pairs of defective TorS proteins was achieved in vitro, allowing TorR transphosphorylation. This strongly suggests that TorS is a multimer in which intermolecular phosphorylation occurs. The wild-type alternative transmitter domain alone was shown to complement a TorS protein mutated in its C-terminal alternative transmitter. Interestingly, overproduction of the alternative transmitter domain led to in vivo TorR-dependent constitutive expression of the tor operon in a torS+ or torS context. Hence, the TorS alternative transmitter contains the phosphodonor site for TorR. Taken together, our results support a TorS phosphorylation cascade from the classical transmitter to the sensor receiver and the alternative transmitter phosphorylation sites.

A bacterial basic region leucine zipper histidine kinase regulating toluene degradation

The two-component signal transduction pathways in bacteria use a histidine-aspartate phosphorelay circuit to mediate cellular changes in response to environmental stimuli. Here we describe a novel two-component todST system, which activates expression of the toluene degradation (tod) pathway in Pseudomonas putida F1. The todS gene is predicted to encode a sensory hybrid kinase with two unique properties--a basic region leucine zipper dimerization motif at the N terminus and a duplicated histidine kinase motif. Evidence from a synthetic peptide model suggests that TodS binds as a dimer to a pseudopalindromic sequence (5'-TGACTCA), which resembles the recognition sequence of the eukaryotic transcription factors Fos and Jun. These results provide additional evidence that bacteria and eukaryotes share common regulatory motifs. The todT gene product, a response regulator, was overproduced as a fusion protein in Escherichia coli, and the purified protein was found to bind specifically to a 6-bp palindromic DNA structure in the tod control region. The phosphorylated form of TodT appears to be the activator of tod structural genes. This is the first report of a two-component system that regulates aromatic metabolism in bacteria.

Central role of the BvgS receiver as a phosphorylated intermediate in a complex two-component phosphorelay

Two-component systems use phosphorylation reactions to regulate stimulus/response pathways. In Bordetella pertussis, a human respiratory pathogen, the infectious cycle of the organism is controlled by the BvgAS two-component system. BvgS has similarities to sensor and response regulator components and is an autophosphorylating kinase that phosphorylates BvgA. BvgA, a response regulator, is a DNA-binding protein that activates virulence gene transcription. Three phosphorylated BvgS domains, the transmitter, receiver, and C terminus, are essential for signal transduction. We now demonstrate that the BvgS transmitter is sufficient for autophosphorylation but is unable to phosphorylate the C terminus or BvgA. The BvgS receiver regulates several phenotypes: dephosphorylation of both the BvgS transmitter and C terminus as well as transfer of a phosphoryl group from the transmitter to the C terminus. Our results indicate that BvgAS signal transduction initiates with autophosphorylation of the transmitter followed by phosphotransfer to the receiver. The phosphorylated receiver can donate to the C terminus or to water. The phosphorylated C terminus is then able to transfer the phosphoryl group to BvgA.

Complex regulatory activities associated with the histidine kinase PrrB in expression of photosynthesis genes in Rhodobacter sphaeroides 2.4.1

Rhodobacter sphaeroides 2.4.1 synthesizes a specialized photosynthetic membrane upon reduction of the O2 tension below threshold levels. The genes prrB and prrA encode a sensor kinase and a response regulator, respectively, of a two-component regulatory system that presumably is involved in transduction of the signal(s) that monitors alterations in oxygen levels. A third gene, prrC, is also involved in this cascade of events. Previously, we described a mutant form of PrrB, namely, PrrB78 (J. M. Eraso and S. Kaplan, J. Bacteriol. 177:2695-2706, 1995), which results in aerobic expression of the photosynthetic apparatus. Here we examine three mutated forms of the prrB gene that have the potential to encode truncated polypeptides containing the N-terminal 6, 63, or 163 amino acids, respectively. The resulting mutant strains showed residual levels of the light-harvesting spectral complexes and had diminished photosynthetic growth rates at high light intensities with no discernible growth under intermediate or low light conditions. When either lacZ transcriptional fusions or direct mRNA determinations were used to monitor specific photosynthesis gene expression, all the mutant strains showed unexpectedly high levels of gene expression when compared to mutant strains affected in prrA. Conversely, when translational fusions were used to monitor photosynthesis gene expression in these mutant strains, expression of both puc and puf operons was reduced, especially puf expression. In light of these studies and those of the PrrB78 mutant, the data suggest that PrrA can be activated in situ by something other than PrrB, and it also appears that PrrB can function as a negative regulator acting through PrrA. Finally, we consider the role of the Prr regulatory system in the posttranscriptional control of photosynthesis gene expression.

Transcriptional control mediated by the ArcA two-component response regulator protein of Escherichia coli: characterization of DNA binding at target promoters

ArcA protein bearing an amino-terminal, oligohistidine extension has been purified, and its DNA binding activity has been characterized with or without prior incubation with carbamoyl phosphate. Electrophoretic mobility shift assays and DNase I protection assays indicate that where the phosphorylated form of the ArcA protein (ArcA-P) is expected to act as a transcriptional repressor (e.g., of lctPRD and gltA-sdhCDAB), the effect is likely to be mediated by sequestration of cis-controlling transcriptional regulatory elements. In contrast, in the case of cydAB, for which ArcA-P is expected to function as a transcriptional activator, two discrete binding sites have been identified upstream of a known promoter, and activation from these sites is likely to be mediated by a mechanism typical of the type I class of prokaryotic transcriptional activators. An additional ArcA-P binding site has also been located downstream of the known promoter, and a distinct role for this site in the regulation of the cydAB operon during anoxic growth transitions is suggested. These results are discussed within the framework of an overall model of signaling by the Arc two-component signal transduction system in response to changes in aerobiosis.

Characterization of an upstream activation sequence and two Rox1p-responsive sites controlling the induction of the yeast HEM13 gene by oxygen and heme deficiency

The Saccharomyces cerevisiae HEM13 gene codes for coproporphyrinogen oxidase, an oxygen-requiring enzyme catalyzing the sixth step of heme biosynthesis. Its transcription has been shown to be induced 40-50-fold in response to oxygen or heme deficiency, in part through relief of repression exerted by Rox1p and in part by activation mediated by an upstream activation sequence (UAS). This report describes an analysis of HEM13 UAS and of the Rox1p-responsive sites by electrophoretic mobility shift assays, DNase I footprinting, and mutational mapping. HEM13 UAS is composed of two subelements: a 16-base pair sequence binding a constitutive factor acting as a transcriptional activator, and a 5'-flanking 20-base pair GC-rich region. Both subelements were required additively for transcription, but each element alone was sufficient for almost normal control by oxygen/heme deficiency. Mutations in both elements decreased the induction ratio 3-4-fold. HEM13 UAS conferred a 2-4-fold oxygen/heme control on a heterologous reporter gene. Two Rox1p-responsive sites, R1 and R3, were identified, which accounted for the 6-7-fold repression by Rox1p. A factor bound to a sequence close to site R3. This DNA-binding activity was only detected in protein extracts of aerobic heme-sufficient ROX1 TUP1 cells, suggesting a possible role in site R3 function.

Yeast HOG1 MAP kinase cascade is regulated by a multistep phosphorelay mechanism in the SLN1-YPD1-SSK1 "two-component" osmosensor

An osmosensing mechanism in the budding yeast (Saccharomyces cerevisiae) involves both a two-component signal transducer (Sln1p, Ypd1p and Ssk1p) and a MAP kinase cascade (Ssk2p/Ssk22p, Pbs2p, and Hog1p). The transmembrane protein Sln1p contains an extracellular sensor domain and cytoplasmic histidine kinase and receiver domains, whereas the cytoplasmic protein Ssk1p contains a receiver domain. Ypd1p binds to both Sln1p and Ssk1p and mediates the multistep phosphotransfer reaction (phosphorelay). This phosphorelay system is initiated by the autophosphorylation of Sln1p at His576. This phosphate is then sequentially transferred to Sln1p-Asp-1144, then to Ypd1p-His64, and finally to Ssk1p-Asp554. We propose that the multistep phosphorelay mechanism is a universal signal transduction apparatus utilized both in prokaryotes and eukaryotes.

Pleiotropic phenotypes caused by genetic ablation of the receiver module of the Agrobacterium tumefaciens VirA protein

The VirA protein of Agrobacterium tumefaciens is a transmembrane sensory kinase that phosphorylates the VirG response regulator in response to chemical signals released from plant wound sites. VirA contains both a two-component kinase module and, at its carboxyl terminus, a receiver module. We previously provided evidence that this receiver module inhibited the activity of the kinase module and that inhibition might be neutralized by phosphorylation. In this report, we provide additional evidence for this model by showing that overexpressing the receiver module in trans can restore low-level basal activity to a VirA mutant protein lacking the receiver module. We also show that ablation of the receiver module restores activity to the inactive VirA (delta324-413) mutant, which has a deletion within a region designated the linker module. This indicates that deletion of the linker module does not denature the kinase module, but rather locks the kinase into a phenotypically inactive conformation, and that this inactivity requires the receiver module. These data provide genetic evidence that the kinase and receiver modules of VirA attain their native conformations autonomously. The receiver module also restricts the variety of phenolic compounds that have stimulatory activity, since removal of this module causes otherwise nonstimulatory phenolic compounds such as 4-hydroxyacetophenone to stimulate vir gene expression.

Mutational analysis of the input domain of the VirA protein of Agrobacterium tumefaciens

The transmembrane sensor protein VirA activates VirG in response to high levels of acetosyringone (AS). In order to respond to low levels of AS, VirA requires the periplasmic sugar-binding protein ChvE and monosaccharides released from plant wound sites. To better understand how VirA senses these inducers, the C58 virA gene was randomly mutagenized, and 14 mutants defective in vir gene induction and containing mutations which mapped to the input domain of VirA were isolated. Six mutants had single missense mutatiions in three widely separated areas of the periplasmic domain. Eight mutants had mutations in or near an amphipathic helix, TM1, or TM2. Four of the mutations in the periplasmic domain, when introduced into the corresponding A6 virA sequence, caused a specific defect in the vir gene response to glucose. This suggests that most of the periplasmic domain is required for the interaction with, or response to, ChvE. Three of the mutations from outside the periplasmic domain, one from each transmembrane domain and one from the amphiphathic helix, were made in A6 virA. These mutants were defective in the vir gene response to AS. These mutations did not affect the stability or topology of VirA or prevent dimerization; therefore, they may interfere with detection of AS or transmission of the signals to the kinase domain. Characterization of C58 chvE mutants revealed that, unlike A6 VirA, C58 VirA requires ChvE for activation of the vir genes.

Effect of microaerophilic cell growth conditions on expression of the aerobic (cyoABCDE and cydAB) and anaerobic (narGHJI, frdABCD, and dmsABC) respiratory pathway genes in Escherichia coli

Escherichia coli varies the synthesis of many of its respiratory enzymes in response to oxygen availability. These enzymes include cytochrome o oxidase (cyoABCDE) and cytochrome d oxidase (cydAB), used during aerobic cell growth, and a fumarate reductase (frdABCD), dimethyl sulfoxide/trimethylamine oxide reductase (dmsABC), and nitrate reductase (narGHJI), used during anaerobic respiratory conditions. To determine how different levels of oxygen affect the expression of each operon, strains containing cyo-lacZ, cyd-lacZ, frdA-lacZ, dmsA-lacZ, and narG-lacZ fusions were grown in continuous culture at various degrees of air saturation. The basal-level expression of the anaerobic respiratory genes, frdABCD, dmsABC, and narGHJI, occurred when the air saturation of the medium was above 20%; as the saturation was reduced to below 10% (ca. 2% oxygen), the expression rapidly increased and reached a maximal level at 0% air. In contrast, cyoABCDE gene expression was lowest under anaerobic conditions while cyd-lacZ expression was about 40% of its maximum level. When the oxygen level was raised into the microaerophilic range (ca. 7% air saturation) cyd-lacZ expression was maximal while cyo-lacZ expression was elevated by about fivefold. As the air level was raised to above 20% saturation, cyd-lacZ expression fell to a basal level while cyo-lacZ expression was increased to its maximum level. The role of the Fnr and ArcA regulatory proteins in this microaerophilic control of respiratory gene expression was documented: whereas Fnr function as an aerobic/anaerobic switch in the range of 0 to 10% air saturation, ArcA exerted its control in the 10 to 20% range. These two transcriptional regulators coordinate the hierarchial control of respiratory pathway gene expression in E. coli to ensure the optimal use of oxygen in the cell environment.

A global regulator of secondary metabolite production in Pseudomonas fluorescens Pf-5

Mutations in the apdA (for antibiotic production) gene of the plant root-colonizing bacterium Pseudomonas fluorescens Pf-5 pleiotropically abolish the production of an array of antibiotics, including pyrrolnitrin, pyoluteorin, and 2,4-diacetylphloroglucinol, as well as the production of tryptophan side chain oxidase, hydrogen cyanide, and an extracellular protease. The lack of production of secondary metabolites by ApdA- mutants was correlated with the loss of inhibition of the phytopathogenic fungus Rhizoctonia solani in culture. Sequencing of the apdA region identified an open reading frame of 2,751 bp. The predicted amino acid sequence of the apdA gene contains conserved domains of the histidine kinases that serve as sensor components of prokaryotic two-component regulatory systems. The apdA nucleotide and predicted amino acid sequences are strikingly similar to the sequences of lemA and repA, genes encoding putative sensor kinases that are required for the pathogenicity of Pseudomonas syringae pv. syringae and Pseudomonas viridiflava, respectively. Introduction of the cloned apdA+ gene restored the wild-type phenotype to both LemA- mutants of P. syringae and ApdA- mutants of Pf-5. The 101-kDa ApdA protein reacted with an anti-LemA antiserum, further demonstrating the similarity of ApdA to LemA. These results show that apdA encodes a putative sensor kinase component of a classical two-component regulatory system that is required for secondary-metabolite production by P. fluorescens Pf-5.

Purification of ArcA and analysis of its specific interaction with the pfl promoter-regulatory region

ArcA is one of several transcription factors required for optimal anaerobic induction of the pyruvate formatelyase (pfl) operon. To aid the study at the molecular level of the interaction of ArcA with the pfl promoter-regulatory region we developed a procedure for the isolation of ArcA. The purification of ArcA involved chromatography in heparin agarose, hydroxylapatite and Mono-Q matrices and delivered a protein that was > 95% pure. Gel retardation assays demonstrated that ArcA bound specifically to the pfl regulatory region. Three distinct ArcA-DNA complexes could be resolved depending on the ArcA concentration used. This finding suggested that either multiple ArcA-binding sites are present in the regulatory region or that ArcA can oligomerize at one or more sites. The DNA-binding activity of ArcA could be increased as estimated 10-fold by prior incubation of the protein with carbamoyl phosphate, suggesting that phosphorylation activates DNA binding or oligomerisation. DNase I footprint analyses identified four sites that were protected by ArcA from cleavage. Two of these sites spanned the transcription start site and -10 regions of promoters 6 and 7, while a third site partially overlapped the characterized binding site of integration host factor (IHF). ArcA exhibited the highest affinity for a stretch of DNA located between the IHF site and the transcription start site of promoter 7. These results are congruent with the hypothesis that a higher-order nucleoprotein complex comprising several proteins, including ArcA, is required to activate transcription from the multiple promoters of the pfl operon.

Histidine and aspartate phosphorylation: two-component systems and the limits of homology

Autophosphorylating histidine kinase and response-regulator domains constitute the building blocks of two-component signaling systems. These systems use a unique phosphotransfer chemistry to regulate many aspects of bacterial physiology. Homologous systems are now being found in eukaryotes. Despite their common mechanism of phosphotransfer, the two-component systems display an extensive diversity in the arrangement of their domains, and flexibility in their roles in different signal transduction circuits.

Regulation of the citrate synthase (gltA) gene of Escherichia coli in response to anaerobiosis and carbon supply: role of the arcA gene product

As an enzyme of the tricarboxylic acid cycle pathway, citrate synthase participates in the generation of a variety of cellular biosynthetic intermediates and in that of reduced purine nucleotides that are used in energy generation via electron transport-linked phosphorylation reactions. It catalyzes the condensation of oxaloacetate and acetyl coenzyme A to produce citrate plus coenzyme A. In Escherichia coli this enzyme is encoded by the gltA gene. To investigate how gltA expression is regulated, a gltA-lacZ operon fusion was constructed and analyzed following aerobic and anaerobic cell growth on various types of culture media. Under aerobic culture conditions, expression was elevated to a level twofold higher than that reached under anaerobic culture conditions. ArcA functions as a repressor of gltA expression under each set of conditions: in a delta arcA strain, gltA-lacZ expression was elevated to levels two- and eightfold higher than those seen in a wild-type strain under aerobic and anaerobic conditions, respectively. This control is independent of the fnr gene product, an alternative anaerobic gene regulator in E. coli. When the richness or type of carbon compound used for cell growth was varied, gltA-lacZ expression varied by 10- to 14-fold during aerobic and anaerobic growth. This regulation was independent of both the crp and fruR gene products, suggesting that another regulatory element in E. coli is responsible for the observed control. Finally, gltA-lacZ expression was shown to be inversely proportional to the cell growth rate. These findings indicate that the regulation of gltA gene expression is complex in meeting the differential needs of the cell for biosynthesis and energy generation under various cell culture conditions.

Aerobic-anaerobic gene regulation in Escherichia coli: control by the ArcAB and Fnr regulons

A variety of pathways for carbon and electron flow in the bacterium Escherichia coli and in other enteric bacteria are differentially expressed depending on whether molecular oxygen is present in the cell environment. This review briefly summarizes the metabolic pathways operative during aerobic versus anaerobic cell growth, and provides a regulatory overview for how the cell controls expression of the many genes involved in these processes. The cell has two distinctly different transcriptional regulators, consisting of the Fnr and the ArcA/ArcB regulatory proteins to accomplish this task. Together, they coordinate gene expression to adjust carbon flow with electron flow and energy generation so that cells can balance growth in an efficiently coupled manner.

Signal transduction. Bringing the eukaryotes up to speed

'Two-component' signalling systems have long been known to be used very generally in bacterial signal transduction; at last, related proteins have been discovered in eukaryotes.

Effects of nitrate respiration on expression of the Arc-controlled operons encoding succinate dehydrogenase and flavin-linked L-lactate dehydrogenase

Expression of sdhCDAB (encoding succinate dehydrogenase) and lctD (encoding the flavin-linked L-lactate dehydrogenase) is elevated aerobically and repressed anaerobically in Escherichia coli. The repression is initiated by autophosphorylation of the sensor protein ArcB, followed by phosphoryl group transfer to the regulator ArcA. ArcA-P, a global transcriptional regulator, then prevents sdh and lct expression. The stimulus for ArcB is not O2 deficiency per se. In vitro experiments showed that ArcB phosphorylation is enhanced by pyruvate, D-lactate, acetate, and NADH, the concentrations of which are likely to increase with the lack of an effective exogenous electron sink. In addition to their aerobic function, the two primary dehydrogenases also have roles in anaerobic nitrate respiration. Results presented here indicate that the increase of sdh and lct expression by nitrate depended on its chemical reduction, which in turn diminished the ArcA-P pool. Unexpectedly, a mutation in the fnr gene (encoding a global regulator involved in anaerobic metabolism) also alleviated the anaerobic repressions. Mutations in arcB or arcA were epistatic over that of fnr. Moreover, since this relief was counteracted by pyruvate in the growth medium, Fnr appears to affect formation of stimuli for ArcB. It is possible that Fnr also indirectly affects some of the other members of the arcA modulon, e.g., cyoABCDE (encoding the cytochrome o complex), cydAB (encoding the cytochrome d complex), and sodA (encoding the manganese-dependent superoxide dismutase).

Oxygen reactions with bacterial oxidases and globins: binding, reduction and regulation

Oxygen is favoured as terminal electron acceptor in aerobic and facultative microorganisms because of its appropriate physical state, satisfactory solubility and its desirable combinations of kinetic and thermodynamic properties. Oxygen is generally reduced by four electrons to yield oxygen, but there are important biological consequences of, and roles for, the partial reduction to superoxide and peroxide. Complex and multiple regulatory networks ensure (i) the utilization of oxygen in preference to other oxidants, (ii) the synthesis of oxygen-consuming enzymes with appropriate properties (particularly affinity for the ligand), and (iii) appropriate cellular protection in the event of oxidative stress. This contribution reviews the terminal respiratory oxidases of selected Gram-negative bacteria and microbial haemoglobin-like proteins. Recent studies of the cytochrome bd-type oxidases of Escherichia coli and Azotobacter vinelandii suggest that, despite probable similarity at the amino acid level, the reactivities of these oxidases with oxygen are strikingly different. The respiratory protection afforded to nitrogenase in the obligately aerobic diazotroph A. vinelandii by the cytochrome bd complex appears to be accompanied by, and may be the result of, a low affinity for oxygen and a high Vmax. The poorly characterized cytochrome o-containing oxidase in this bacterium is not required for respiratory protection. In E. coli, the cytochrome bd-type oxidase has a remarkably high affinity for oxygen, consistent with the view that this is an oxygen-scavenging oxidase utilized under microaerobic conditions. The demonstration of substrate (i.e. oxygen) inhibition in this complex suggests a mechanism whereby wasteful electron flux through a non-proton-pumping oxidase is avoided at higher dissolved oxygen tensions. The demonstration of two ligand-binding sites (haems d and b595) in oxidases of this type suggests plausible mechanisms for this phenomenon. In E. coli, assembly of the cytochrome bd-type oxidase (and of periplasmic cytochromes b and c) requires the presence of an ABC transporter, which may serve to export haem or some "assembly factor" to the periplasm. There is at least one additional oxygen-consuming protein in E. coli-the flavohaemoglobin encoded by the hmp gene. Globin-like proteins are also widely distributed in other bacteria, fungi and protozoa, but most have unknown functions. The function of HMP and the related chimaeric flavohaemoglobins in other bacteria and yeast is unknown; one of several possibilities for HMP is that its relatively low affinity for oxygen during turnover with NADH as substrate could enable it to function as a sensor of failing (or rising) cytoplasmic oxygen concentrations.