Specific overproduction and purification of the cytochrome b558 component of the cytochrome d complex from Escherichia coli

Electrocatalytic evidence of the diversity of the oxygen reaction in the bacterial bd oxidase from different organisms

Cytochrome bd oxidase is a bacterial terminal oxygen reductase that was suggested to enable adaptation to different environments and to confer resistance to stress conditions. An electrocatalytic study of the cyt bd oxidases from Escherichia coli, Corynebacterium glutamicum and Geobacillus thermodenitrificans gives evidence for a different reactivity towards oxygen. An inversion of the redox potential values of the three hemes is found when comparing the enzymes from different bacteria. This inversion can be correlated with different protonated glutamic acids as evidenced by reaction induced FTIR spectroscopy. The influence of the microenvironment of the hemes on the reactivity towards oxygen is discussed.

Oxygen as Acceptor

Like most bacteria, Escherichia coli has a flexible and branched respiratory chain that enables the prokaryote to live under a variety of environmental conditions, from highly aerobic to completely anaerobic. In general, the bacterial respiratory chain is composed of dehydrogenases, a quinone pool, and reductases. Substrate-specific dehydrogenases transfer reducing equivalents from various donor substrates (NADH, succinate, glycerophosphate, formate, hydrogen, pyruvate, and lactate) to a quinone pool (menaquinone, ubiquinone, and dimethylmenoquinone). Then electrons from reduced quinones (quinols) are transferred by terminal reductases to different electron acceptors. Under aerobic growth conditions, the terminal electron acceptor is molecular oxygen. A transfer of electrons from quinol to O₂ is served by two major oxidoreductases (oxidases), cytochrome bo₃ encoded by cyoABCDE and cytochrome bd encoded by cydABX. Terminal oxidases of aerobic respiratory chains of bacteria, which use O₂ as the final electron acceptor, can oxidize one of two alternative electron donors, either cytochrome c or quinol. This review compares the effects of different inhibitors on the respiratory activities of cytochrome bo₃ and cytochrome bd in E. coli. It also presents a discussion on the genetics and the prosthetic groups of cytochrome bo₃ and cytochrome bd. The E. coli membrane contains three types of quinones that all have an octaprenyl side chain (C₄₀). It has been proposed that the bo₃ oxidase can have two ubiquinone-binding sites with different affinities. "WHAT'S NEW" IN THE REVISED ARTICLE: The revised article comprises additional information about subunit composition of cytochrome bd and its role in bacterial resistance to nitrosative and oxidative stresses. Also, we present the novel data on the electrogenic function of appBCX-encoded cytochrome bd-II, a second bd-type oxidase that had been thought not to contribute to generation of a proton motive force in E. coli, although its spectral properties closely resemble those of cydABX-encoded cytochrome bd.

The cytochrome bd respiratory oxygen reductases

Cytochrome bd is a respiratory quinol: O₂ oxidoreductase found in many prokaryotes, including a number of pathogens. The main bioenergetic function of the enzyme is the production of a proton motive force by the vectorial charge transfer of protons. The sequences of cytochromes bd are not homologous to those of the other respiratory oxygen reductases, i.e., the heme-copper oxygen reductases or alternative oxidases (AOX). Generally, cytochromes bd are noteworthy for their high affinity for O₂ and resistance to inhibition by cyanide. In E. coli, for example, cytochrome bd (specifically, cytochrome bd-I) is expressed under O₂-limited conditions. Among the members of the bd-family are the so-called cyanide-insensitive quinol oxidases (CIO) which often have a low content of the eponymous heme d but, instead, have heme b in place of heme d in at least a majority of the enzyme population. However, at this point, no sequence motif has been identified to distinguish cytochrome bd (with a stoichiometric complement of heme d) from an enzyme designated as CIO. Members of the bd-family can be subdivided into those which contain either a long or a short hydrophilic connection between transmembrane helices 6 and 7 in subunit I, designated as the Q-loop. However, it is not clear whether there is a functional consequence of this difference. This review summarizes current knowledge on the physiological functions, genetics, structural and catalytic properties of cytochromes bd. Included in this review are descriptions of the intermediates of the catalytic cycle, the proposed site for the reduction of O₂, evidence for a proton channel connecting this active site to the bacterial cytoplasm, and the molecular mechanism by which a membrane potential is generated.

Oxygen as Acceptor

Like most bacteria, Escherichia coli has a flexible and branched respiratory chain that enables the prokaryote to live under a variety of environmental conditions, from highly aerobic to completely anaerobic. In general, the bacterial respiratory chain is composed of dehydrogenases, a quinone pool, and reductases. Substrate specific dehydrogenases transfer reducing equivalents from various donor substrates (NADH, succinate, glycerophoshate, formate, hydrogen, pyruvate, and lactate) to a quinone pool (menaquinone, ubiquinone, and demethylmenoquinone). Then electrons from reduced quinones (quinols) are transferred by terminal reductases to different electron acceptors. Under aerobic growth conditions, the terminal electron acceptor is molecular oxygen. A transfer of electrons from quinol to O2 is served by two major oxidoreductases (oxidases), cytochrome bo3 and cytochrome bd. Terminal oxidases of aerobic respiratory chains of bacteria, which use O2 as the final electron acceptor, can oxidize one of two alternative electron donors, either cytochrome c or quinol. This review compares the effects of different inhibitors on the respiratory activities of cytochrome bo3 and cytochrome bd in E. coli. It also presents a discussion on the genetics and the prosthetic groups of cytochrome bo3 and cytochrome bd. The E. coli membrane contains three types of quinones which all have an octaprenyl side chain (C40). It has been proposed that the bo3 oxidase can have two ubiquinone-binding sites with different affinities. The spectral properties of cytochrome bd-II closely resemble those of cydAB-encoded cytochrome bd.

Heme/heme redox interaction and resolution of individual optical absorption spectra of the hemes in cytochrome bd from Escherichia coli

Cytochrome bd is a terminal component of the respiratory chain of Escherichia coli catalyzing reduction of molecular oxygen to water. It contains three hemes, b(558), b(595), and d. The detailed spectroelectrochemical redox titration and numerical modeling of the data reveal significant redox interaction between the low-spin heme b(558) and high-spin heme b(595), whereas the interaction between heme d and either hemes b appears to be rather weak. However, the presence of heme d itself decreases much larger interaction between the two hemes b. Fitting the titration data with a model where redox interaction between the hemes is explicitly included makes it possible to extract individual absorption spectra of all hemes. The alpha- and beta-band reduced-minus-oxidized difference spectra agree with the data published earlier ([22] J.G. Koland, M.J. Miller, R.B. Gennis, Potentiometric analysis of the purified cytochrome d terminal oxidase complex from Escherichia coli, Biochemistry 23 (1984) 1051-1056., and [23] R.M. Lorence, J.G. Koland, R.B. Gennis, Coulometric and spectroscopic analysis of the purified cytochrome d complex of Escherichia coli: evidence for the identification of "cytochrome a(1)" as cytochrome b(595), Biochemistry 25 (1986) 2314-2321.). The Soret band spectra show lambda(max)=429.5 nm, lambda(min) approximately 413 nm (heme b(558)), lambda(max)=439 nm, lambda(min) approximately 400+/-1 nm (heme b(595)), and lambda(max)=430 nm, lambda(min)=405 nm (heme d). The spectral contribution of heme d to the complex Soret band is much smaller than those of either hemes b; the Soret/alpha (DeltaA(430):DeltaA(629)) ratio for heme d is 1.6.

Molecular mechanism of proton translocation by cytochrome c oxidase

Cytochrome c oxidase (CcO) is a terminal protein of the respiratory chain in eukaryotes and some bacteria. It catalyzes most of the biologic oxygen consumption on earth done by aerobic organisms. During the catalytic reaction, CcO reduces dioxygen to water and uses the energy released in this process to maintain the electrochemical proton gradient by functioning as a redox-linked proton pump. Even though the structures of several terminal oxidases are known, they are not sufficient in themselves to explain the molecular mechanism of proton pumping. Thus, additional extensive studies of CcO by varieties of biophysical and biochemical approaches are involved to shed light on the mechanism of proton translocation. In this review, we summarize the current level of knowledge about CcO, including the latest model developed to explain the CcO proton-pumping mechanism.

Asymmetrical Evolution of Cytochrome bd Subunits

Functionally linked genes generally evolve at similar rates and the knowledge of this particular feature of genomic evolution has been used as the basis for the phylogenetic profiling method. We illustrate here an exception to this rule in the evolution of the cytochrome bd complex. This is a two-component oxidase complex, with the subunits I and II known to be widely present in bacteria. The subunits within the cytochrome bd complex are under the same evolutionary pressure and most likely behave in the same evolutionary manner. However, the sequence similarity of genes encoding subunit II varies considerably across species. Genes encoding subunit II evolve 1.2 times faster on most of the branches of their phylogeny than subunit I genes. Furthermore, the genes encoding subunit II in Oceanobacillus iheyensis, Bacillus halodurans, and Staphylococcus species do not have detectable homologues within E. coli due to their large divergence. Together, the two subunits of cytochrome bd reveal an interesting example of an asymmetric pattern of evolutionary change.

Interaction of cytochrome bd with carbon monoxide at low and room temperatures: evidence that only a small fraction of heme b595 reacts with CO

Azotobacter vinelandii is an obligately aerobic bacterium in which aerotolerant dinitrogen fixation requires cytochrome bd. This oxidase comprises two polypeptide subunits and three hemes, but no copper, and has been studied extensively. However, there remain apparently conflicting reports on the reactivity of the high spin heme b(595) with ligands. Using purified cytochrome bd, we show that absorption changes induced by CO photodissociation from the fully reduced cytochrome bd at low temperatures demonstrate binding of the ligand with heme b(595). However, the magnitude of these changes corresponds to the reaction with CO of only about 5% of the heme. CO binding with a minor fraction of heme b(595) is also revealed at room temperature by time-resolved studies of CO recombination. The data resolve the apparent discrepancies between conclusions drawn from room and low temperature spectroscopic studies of the CO reaction with cytochrome bd. The results are consistent with the proposal that hemes b(595) and d form a diheme oxygen-reducing center with a binding capacity for a single exogenous ligand molecule that partitions between the hemes d and b(595) in accordance with their intrinsic affinities for the ligand. In this model, the affinity of heme b(595) for CO is about 20-fold lower than that of heme d.

Gene structure and quinol oxidase activity of a cytochrome bd-type oxidase from Bacillus stearothermophilus

Gram-positive thermophilic Bacillus species contain cytochrome caa3-type cytochrome c oxidase as their main terminal oxidase in the respiratory chain. We previously identified and purified an alternative oxidase, cytochrome bd-type quinol oxidase, from a mutant of Bacillus stearothermophilus defective in the caa3-type oxidase activity (J. Sakamoto et al., FEMS Microbiol. Lett. 143 (1996) 151-158). Compared with proteobacterial counterparts, B. stearothermophilus cytochrome bd showed lower molecular weights of the two subunits, shorter wavelength of alpha-band absorption maximum due to heme D, and lower quinol oxidase activity. Preincubation with menaquinone-2 enhanced the enzyme activity up to 40 times, suggesting that, besides the catalytic site, there is another quinone-binding site which largely affects the enzyme activity. In order to clarify the molecular basis of the differences of cytochromes bd between B. stearothermophilus and proteobacteria, the genes encoding for the B. stearothermophilus bd was cloned based on its partial peptide sequences. The gene for subunit I (cbdA) encodes 448 amino acid residues with a molecular weight of 50195 Da, which is 14 and 17% shorter than those of Escherichia coli and Azotobacter vinelandii, respectively, and CbdA lacks the C-terminal half of the long hydrophilic loop between the putative transmembrane segments V and VI (Q loop), which has been suggested to include the substrate quinone-binding site for the E. coli enzyme. The gene for subunit II (cbdB) encodes 342 residues with a molecular weight of 38992 Da. Homology search indicated that the B. stearothermophilus cbdAB has the highest sequence similarity to ythAB in B. subtilis genome rather than to cydAB, the first set of cytochrome bd genes identified in the genome. Sequence comparison of cytochromes bd and their homologs from various organisms demonstrates that the proteins can be classified into two subfamilies, a proteobacterial type including E. coli bd and a more widely distributed type including the B. stearothermophilus enzyme, suggesting that the latter type is evolutionarily older.

Sequence analysis of cytochrome bd oxidase suggests a revised topology for subunit I

Numerous sequences of the cytochrome bd quinol oxidase (cytochrome bd) have recently become available for analysis. The analysis has revealed a small number of conserved residues, a new topology for subunit I and a phylogenetic tree involving extensive horizontal gene transfer. There are 20 conserved residues in subunit I and two in subunit II. Algorithms utilizing multiple sequence alignments predicted a revised topology for cytochrome bd, adding two transmembrane helices to subunit I to the seven that were previously indicated by the analysis of the sequence of the oxidase from E. coli. This revised topology has the effect of relocating the N-terminus and C-terminus to the periplasmic and cytoplasmic sides of the membrane, respectively. The new topology repositions I-H19, the putative ligand for heme b595, close to the periplasmic edge of the membrane, which suggests that the heme b595/heme d active site of the oxidase is located near the outer (periplasmic) surface of the membrane. The most highly conserved region of the sequence of subunit I contains the sequence GRQPW and is located in a predicted periplasmic loop connecting the eighth and ninth transmembrane helices. The potential importance of this region of the protein was previously unsuspected, and it may participate in the binding of either quinol or heme d. There are two very highly conserved glutamates in subunit I, E99 and E107, within the third transmembrane helix (E. coli cytochrome bd-I numbering). It is speculated that these glutamates may be part of a proton channel leading from the cytoplasmic side of the membrane to the heme d oxygen-reactive site, now placed near the periplasmic surface. The revised topology and newly revealed conserved residues provide a clear basis for further experimental tests of these hypotheses. Phylogenetic analysis of the new sequences of cytochrome bd reveals considerable deviation from the 16sRNA tree, suggesting that a large amount of horizontal gene transfer has occurred in the evolution of cytochrome bd.

Linkage map of Escherichia coli K-12, edition 10: the traditional map

This map is an update of the edition 9 map by Berlyn et al. (M. K. B. Berlyn, K. B. Low, and K. E. Rudd, p. 1715-1902, in F. C. Neidhardt et al., ed., Escherichia coli and Salmonella: cellular and molecular biology, 2nd ed., vol. 2, 1996). It uses coordinates established by the completed sequence, expressed as 100 minutes for the entire circular map, and adds new genes discovered and established since 1996 and eliminates those shown to correspond to other known genes. The latter are included as synonyms. An alphabetical list of genes showing map location, synonyms, the protein or RNA product of the gene, phenotypes of mutants, and reference citations is provided. In addition to genes known to correspond to gene sequences, other genes, often older, that are described by phenotype and older mapping techniques and that have not been correlated with sequences are included.

Cytochrome bd oxidase from Azotobacter vinelandii. Purification and quantitation of ligand binding to the oxygen reduction site

Cytochrome bd has been purified from Azotobacter vinelandii by a new simplified procedure. The heme and total iron content has been measured, as has the number of high affinity CO and NO binding sites. Spectral changes indicate high affinity binding of CO and NO to heme d only, with a stoichiometry of 1 molecule of gas per 2 molecules of heme b or per 3 atoms of iron. The results clearly define a stoichiometry of one heme d per complex. Low affinity binding of CO and NO to heme b595 also occurs at higher ligand concentrations. EPR heme-nitrosyl signals are seen with NO bound to both hemes b595 and d but with no indication of spin exchange coupling. Exposure of the air-oxidized complex to alkaline pH results in removal of molecular oxygen from heme d and a change in line shape of the high spin region of the EPR spectrum. Cyanide binds to both heme d and heme b595 in the air-oxidized complex, displacing molecular oxygen from heme d. The rate of cyanide binding to heme d as assessed by spectral changes at 650 nm does not correlate with the rate of binding to heme b595 as assessed by the loss of the high spin EPR signal. In addition, the cyanide binding rate in the presence of reductant is only 3 times that of the rate of binding to the air-oxidized enzyme, in contrast to the copper-containing oxidases where strong redox cooperativity makes these two rates differ by a factor of at least 10(6). The results do not support the idea of the presence of two strongly interacting hemes in a binuclear center.

The haem b558 component of the cytochrome bd quinol oxidase complex from Escherichia coli has histidine-methionine axial ligation

The cytochrome bd ubiquinol oxidase from Escherichia coli is induced when the bacteria are cultured under microaerophilic or low-aeration conditions. This membrane-bound respiratory oxidase catalyses the two-electron oxidation of ubiquinol and the four-electron reduction of dioxygen to water. The oxidase contains three haem prosthetic groups: haem b558, haem b595 and haem d. Haem d is the oxygen binding site, and it is likely that haem d and b595 form a bimetallic site in the enzyme. Haem b558 has been previously characterized spectroscopically as being low spin and has been shown to be located within subunit I (CydA) of this two-subunit enzyme. It is likely that haem b558 is associated with the quinol oxidation site, which has also been shown to be within subunit I. In a previous effort to locate the specific amino acids axially ligated to haem b558, all six histidines within subunit I were altered by site-directed mutagenesis. Only one, histidine-186, was identified as a likely ligand to haem b558. Hence it was suggested that haem b558 could not have bis(histidine) ligation. In the current work, a combination of low-temperature near-infrared magnetic circular dichroism (NIR-MCD) and EPR spectroscopies have been employed to identify the nature of the haem b558 axial ligands. The NIR-MCD spectrum at cryogenic temperatures is dominated by the low-spin haem b558 component of the complex, and the low-energy band near 1800 nm is strong evidence for histidine-methionine ligation. It is concluded that haem b558 is ligated to histidine-186 plus one of the methionines located within subunit I of the oxidase.

Antimycin inhibition of the cytochrome bd complex from Azotobacter vinelandii indicates the presence of a branched electron transfer pathway for the oxidation of ubiquinol

Antimycin A and UHBDT inhibit the activity of the purified cytochrome bd complex from Azotobacter vinelandii. Inhibition of activity is non-competitive and antimycin A binding induces a shift to the red in the spectrum of a b-type haem. No inhibitory effects were seen with myxothiazol. Steady-state experiments indicate that the site of inhibition for antimycin A lies on the low-potential side of haem b558. In the presence of antimycin A at concentrations sufficient to inhibit respiration, some direct electron transfer from ubiquinol-1 to haem b595 and haem d still occurs. The results are consistent with a branched electron transfer pathway from ubiquinol to the oxygen reduction site.

Spectroscopic evidence for a heme-heme binuclear center in the cytochrome bd ubiquinol oxidase from Escherichia coli

The cytochrome bd complex is a ubiquinol oxidase, which is part of the aerobic respiratory chain of Escherichia coli. This enzyme is structurally unrelated to the heme-Cu oxidases such as cytochrome c oxidase. While the cytochrome bd complex contains no copper, it does have three heme prosthetic groups: heme b558, heme b595, and heme d (a chlorin). Heme b558 appears to be involved in the oxidation of quinol, and heme d is known to be the site where oxygen binds and is reduced to water. The role of heme b595, which is high spin, is not known. In this paper, CO is used to probe the oxygen-binding site by use of Fourier transform infrared spectroscopy to monitor the stretching frequency of CO bound to the enzyme. Photodissociation at low temperature (e.g., 20 K) of the CO-heme d adduct results in CO associated with the protein within the heme binding pocket. This photodissociated CO can subsequently relax to form a kinetically trapped CO-heme b595 adduct. The data clearly show that heme d and heme b595 must reside within a common binding pocket in the enzyme. The catalytic active site where oxygen is reduced to water is, thus, properly considered to be a heme d-heme b595 binuclear center. This is analogous to the heme alpha 3-Cu(B) binuclear center in the heme-Cu oxidases. Heme b595 may play roles analogous to those proposed for the Cu(B) component of cytochrome c oxidase.

Isolation and characterization of a new class of cytochrome d terminal oxidase mutants of Escherichia coli

Cytochrome d terminal oxidase mutants were isolated by using hydroxylamine mutagenesis of pNG2, a pBR322-derived plasmid containing the wild-type cyd operon. The mutagenized plasmid was transformed into a cyo cyd recA strain, and the transformants were screened for the inability to confer aerobic growth on nonfermentable carbon sources. Western blot analysis and visible-light spectroscopy were performed to characterize three independent mutants grown both aerobically and anaerobically. The mutational variants of the cytochrome d complex were stabilized under anaerobic growth conditions. All three mutations perturb the b595 and d heme components of the complex. These mutations were mapped and sequenced and are shown to be located in the N-terminal third of subunit II of the cytochrome d complex. It is proposed that the N terminus of subunit II may interact with subunit I to form an interface that binds the b595 and d heme centers.

Proteolysis of the cytochrome d complex with trypsin and chymotrypsin localizes a quinol oxidase domain

The cytochrome d complex is a two-subunit, membrane-bound terminal oxidase in the aerobic respiratory chain of Escherichia coli. The enzyme catalyzes the two-electron oxidation of ubiquinol and the four-electron reduction of oxygen to water. Previous work demonstrated that the site for ubiquinol oxidation was selectively inactivated by limited proteolysis by trypsin, which cleaves at a locus within subunit I. This work is extended to show that a similar phenomenon is observed with limited chymotrypsin proteolysis of the complex. The cleavage patterns are similar whether one uses the purified oxidase in nondenaturing detergent or reconstituted in proteoliposomes or uses spheroplasts of E. coli as the substrate for the proteolysis. Hence, the protease-sensitive locus is periplasmic in the cell. Fragments resulting from proteolysis were characterized by N-terminal sequencing and by immunoblotting with the use of a monoclonal antibody of known epitope within subunit I. The data indicate that inactivation of the ubiquinol oxidase activity results from cleavage at specific residues with a hydrophilic region previously defined as the Q loop. This domain has been already implicated in ubiquinol oxidation by the use of inhibitory monoclonal antibodies. Electrochemical and HPLC analysis of the protease-cleaved oxidase suggests no global changes in either the quaternary or tertiary structure of the enzyme. It is likely that the Q loop is directly involved in forming a portion of the ubiquinol binding site near the periplasmic surface of the membrane.

The cytochromes of anaerobically grown Escherichia coli. An electron-paramagnetic-resonance study of the cytochrome bd complex in situ

The e.p.r. signals attributable to a cytochrome bd-type ubiquinol:O2 oxidoreductase (cytochrome b-558-b-595-d) were studied in a cytoplasmic membrane preparation of Escherichia coli that had been grown on glycerol with fumarate as respiratory-chain oxidant. Two major high-spin ferric haem signals were resolved on the basis of their potentiometric behaviour: a rhombic high-spin species (gx = 6.25, gy = 5.54) was assigned to haem b-595, and an axial high-spin (gx = 5.97, gy = 5.96) species was assigned to the haem d. These signals titrated with Em.7 values of 154 and 261 mV respectively, corresponding closely to optically determined values for haem b-595 and haem d. At high potentials (greater than 300 mV) the rhombic species attributable to haem b-595 underwent a partial transition to a second rhombic species with g-values of 6.24 (gx) and 5.67 (gy). The high-spin ferric haem spectra were affected by O2, CO, cyanide and pH. A low-spin ferric haem signal was observed at g = 3.3 (gz), which titrated with an Em.7 of 226 mV, and this was assigned to haem b-558. The data support a model for cytochrome bd with two ligand-binding sites, a single haem d and a single haem b-595.

EPR studies of the cytochrome-d complex of Escherichia coli

We have examined the thermodynamic and EPR properties of one of the ubiquinol oxidase systems (the cytochrome d complex) of Escherichia coli, and have assigned the EPR-detectable signals to the optically identified cytochromes. The axial high spin g = 6.0 signal has been assigned to cytochrome d based on the physicochemical properties of this signal and those of the optically defined cytochrome d. A rhombic low spin species at gx,y,z = 1.85, 2.3, 2.5 exhibited similar properties but was present at only one-fifth the concentration of the axial high spin species. Both species have an Em7 of 260 mV and follow a -60 mV/pH unit dependence from pH 6 to 10. The rhombic high spin signal with gy,z = 5.5 and 6.3 has been assigned to cytochrome b-595. This component has an Em7 of 136 mV and follows a -30 mV/pH unit dependence from pH 6 to 10. Lastly, the low spin gz = 3.3 signal which titrates with an Em7 of 195 mV and follows a -40 mV/pH unit dependence from pH 6 to 10 has been assigned to cytochrome b-558. Spin quantitation of the high-spin signals indicates that cytochrome d and b-595 are present in approximately equal amounts. These observations are discussed in terms of the stoichiometry of the prosthetic groups and its implications on the mechanism of electron transport.

Genetic analysis of the membrane insertion and topology of MalF, a cytoplasmic membrane protein of Escherichia coli

MalF is an essential cytoplasmic membrane protein of the maltose transport system of Escherichia coli. We have developed a general approach for analysis of the mechanism of integration of membrane proteins and their membrane topology by characterizing a series of fusions of beta-galactosidase to MalF. The properties of the fusion proteins indicate the following. (1) The first two presumed transmembrane segments of MalF are sufficient to anchor beta-galactosidase firmly to the inner membrane. (2) Hybrid proteins with beta-galactosidase fused to a presumed cytoplasmic domain of MalF have high beta-galactosidase specific activity; fusions to periplasmic domains have low activity. We propose therefore, that periplasmic and cytoplasmic domains of integral membrane proteins can be distinguished by the enzymatic properties of such hybrid proteins. In general, it appears that cleaved or non-cleaved signal sequences when attached to beta-galactosidase cause it to become embedded in the membrane, and this results in the inability of the hybrid proteins to assemble into active enzyme. Additional properties of these fusion proteins contribute to our understanding of the regulation of MalF synthesis. The MalF protein, synthesized as part of the malEFG operon of E. coli, is approximately 30-fold less abundant in the cell than MalE protein (the maltose-binding protein). Differential amounts of the fusion proteins indicate that a regulatory signal occurs within the malF gene that is responsible for the step-down in expression from the malE gene to the malF gene.

Escherichia coli strains carrying the cloned cytochrome d terminal oxidase complex are sensitive to near-UV inactivation

To determine if membrane-bound cytochromes function as endogenous near-UV photosensitizers, strains containing the cloned cydA and cydB genes were tested for near-UV sensitivity. A strain containing both cloned genes overproduced cytochromes b558, b595, and d. Another strain containing only cloned cydB overproduced cytochrome b558. Both cytochrome-overproducing strains were hypersensitive to broad-spectrum near-UV inactivation. The presence of excess cytochromes did not affect sensitivity to far-UV radiation and provided protection against H2O2 inactivation.

Identification of the cydC locus required for expression of the functional form of the cytochrome d terminal oxidase complex in Escherichia coli

The aerobic respiratory chain of Escherichia coli contains two terminal oxidases which are differentially regulated. The cytochrome o complex predominates under growth conditions of high aeration, whereas the cytochrome d complex predominates when the oxygen tension is low. Either terminal oxidase will support aerobic growth. The goal of the work presented in this paper was to identify genes required for the expression of the functional form of the cytochrome d complex, other than the genes encoding the polypeptide components of the oxidase complex (cyd locus). A strain lacking the cytochrome o complex (cyo mutant strain) was mutagenized by using a lambda-Mu hybrid hopper bacteriophage, lambda placMu53, which inserts randomly into the chromosome and carries a kanamycin resistance marker. Strains were isolated and examined which were unable to grow aerobically, i.e., which lacked functional cytochrome d complex, and which could not be complemented by introduction of the cyd gene on F-prime episomes. One strain was selected for characterization. The phage insert was mapped to min 18.9 on the genetic linkage map, defining a new genetic locus, cydC. Evidence described in the text suggests that the gene product is probably required for the synthesis of the unique heme d component of the cytochrome d complex.