Genetic screening in Saccharomyces cerevisiae for large numbers of mitochondrial point mutations which affect structure and function of catalytic subunits of cytochrome-c oxidase

Electron Transfer Coupled to Conformational Dynamics in Cell Respiration

Cellular respiration is a fundamental process required for energy production in many organisms. The terminal electron transfer complex in mitochondrial and many bacterial respiratory chains is cytochrome c oxidase (CcO). This converts the energy released in the cytochrome c/oxygen redox reaction into a transmembrane proton electrochemical gradient that is used subsequently to power ATP synthesis. Despite detailed knowledge of electron and proton transfer paths, a central question remains as to whether the coupling between electron and proton transfer in mammalian mitochondrial forms of CcO is mechanistically equivalent to its bacterial counterparts. Here, we focus on the conserved span between H376 and G384 of transmembrane helix (TMH) X of subunit I. This conformationally-dynamic section has been suggested to link the redox activity with the putative H pathway of proton transfer in mammalian CcO. The two helix X mutants, Val380Met (V380M) and Gly384Asp (G384D), generated in the genetically-tractable yeast CcO, resulted in a respiratory-deficient phenotype caused by the inhibition of intra-protein electron transfer and CcO turnover. Molecular aspects of these variants were studied by long timescale atomistic molecular dynamics simulations performed on wild-type and mutant bovine and yeast CcOs. We identified redox- and mutation-state dependent conformational changes in this span of TMH X of bovine and yeast CcOs which strongly suggests that this dynamic module plays a key role in optimizing intra-protein electron transfers.

A common coupling mechanism for A-type heme-copper oxidases from bacteria to mitochondria

We present a comprehensive investigation of mitochondrial DNA-encoded variants of cytochrome c oxidase (CcO) that harbor mutations within their core catalytic subunit I, designed to interrogate the presently disputed functions of the three putative proton channels. We assess overall respiratory competence, specific CcO catalytic activity, and, most importantly, proton/electron (H⁺/e⁻) stoichiometry from adenosine diphosphate to oxygen ratio measurements on preparations of intact mitochondria. We unequivocally show that yeast mitochondrial CcO uses the D-channel to translocate protons across its hydrophilic core, providing direct evidence in support of a common proton pumping mechanism across all members of the A-type heme-copper oxidase superfamily, independent of their bacterial or mitochondrial origin.

Mitochondria metabolize almost all the oxygen that we consume, reducing it to water by cytochrome c oxidase (CcO). CcO maximizes energy capture into the protonmotive force by pumping protons across the mitochondrial inner membrane. Forty years after the H⁺/e⁻ stoichiometry was established, a consensus has yet to be reached on the route taken by pumped protons to traverse CcO’s hydrophobic core and on whether bacterial and mitochondrial CcOs operate via the same coupling mechanism. To resolve this, we exploited the unique amenability to mitochondrial DNA mutagenesis of the yeast Saccharomyces cerevisiae to introduce single point mutations in the hydrophilic pathways of CcO to test function. From adenosine diphosphate to oxygen ratio measurements on preparations of intact mitochondria, we definitely established that the D-channel, and not the H-channel, is the proton pump of the yeast mitochondrial enzyme, supporting an identical coupling mechanism in all forms of the enzyme.

The Cox1 C-terminal domain is a central regulator of cytochrome c oxidase biogenesis in yeast mitochondria

Cytochrome c oxidase (CcO) is the last electron acceptor in the respiratory chain. The CcO core is formed by mitochondrial DNA-encoded Cox1, Cox2, and Cox3 subunits. Cox1 synthesis is highly regulated; for example, if CcO assembly is blocked, Cox1 synthesis decreases. Mss51 activates translation of COX1 mRNA and interacts with Cox1 protein in high-molecular-weight complexes (COA complexes) to form the Cox1 intermediary assembly module. Thus, Mss51 coordinates both Cox1 synthesis and assembly. We previously reported that the last 15 residues of the Cox1 C terminus regulate Cox1 synthesis by modulating an interaction of Mss51 with Cox14, another component of the COA complexes. Here, using site-directed mutagenesis of the mitochondrial COX1 gene from Saccharomyces cerevisiae, we demonstrate that mutations P521A/P522A and V524E disrupt the regulatory role of the Cox1 C terminus. These mutations, as well as C terminus deletion (Cox1ΔC15), reduced binding of Mss51 and Cox14 to COA complexes. Mss51 was enriched in a translationally active form that maintains full Cox1 synthesis even if CcO assembly is blocked in these mutants. Moreover, Cox1ΔC15, but not Cox1-P521A/P522A and Cox1-V524E, promoted formation of aberrant supercomplexes in CcO assembly mutants lacking Cox2 or Cox4 subunits. The aberrant supercomplex formation depended on the presence of cytochrome b and Cox3, supporting the idea that supercomplex assembly factors associate with Cox3 and demonstrating that supercomplexes can be formed even if CcO is inactive and not fully assembled. Our results indicate that the Cox1 C-terminal end is a key regulator of CcO biogenesis and that it is important for supercomplex formation/stability.

Assembly factors monitor sequential hemylation of cytochrome b to regulate mitochondrial translation

Mitochondrial respiratory chain complexes convert chemical energy into a membrane potential by connecting electron transport with charge separation. Electron transport relies on redox cofactors that occupy strategic positions in the complexes. How these redox cofactors are assembled into the complexes is not known. Cytochrome b, a central catalytic subunit of complex III, contains two heme bs. Here, we unravel the sequence of events in the mitochondrial inner membrane by which cytochrome b is hemylated. Heme incorporation occurs in a strict sequential process that involves interactions of the newly synthesized cytochrome b with assembly factors and structural complex III subunits. These interactions are functionally connected to cofactor acquisition that triggers the progression of cytochrome b through successive assembly intermediates. Failure to hemylate cytochrome b sequesters the Cbp3-Cbp6 complex in early assembly intermediates, thereby causing a reduction in cytochrome b synthesis via a feedback loop that senses hemylation of cytochrome b.

Yeast cytochrome c oxidase: a model system to study mitochondrial forms of the haem-copper oxidase superfamily

The known subunits of yeast mitochondrial cytochrome c oxidase are reviewed. The structures of all eleven of its subunits are explored by building homology models based on the published structures of the homologous bovine subunits and similarities and differences are highlighted, particularly of the core functional subunit I. Yeast genetic techniques to enable introduction of mutations into the three core mitochondrially-encoded subunits are reviewed.

The carboxyl-terminal end of Cox1 is required for feedback assembly regulation of Cox1 synthesis in Saccharomyces cerevisiae mitochondria

Synthesis of the largest cytochrome c oxidase (CcO) subunit, Cox1, on yeast mitochondrial ribosomes is coupled to assembly of CcO. The translational activator Mss51 is sequestered in early assembly intermediate complexes by an interaction with Cox14 that depends on the presence of newly synthesized Cox1. If CcO assembly is prevented, the level of Mss51 available for translational activation is reduced. We deleted the C-terminal 11 or 15 residues of Cox1 by site-directed mutagenesis of mtDNA. Although these deletions did not prevent respiratory growth of yeast, they eliminated the assembly-feedback control of Cox1 synthesis. Furthermore, these deletions reduced the strength of the Mss51-Cox14 interaction as detected by co-immunoprecipitation, confirming the importance of the Cox1 C-terminal residues for Mss51 sequestration. We surveyed a panel of mutations that block CcO assembly for the strength of their effect on Cox1 synthesis, both by pulse labeling and expression of the ARG8(m) reporter fused to COX1. Deletion of the nuclear gene encoding Cox6, one of the first subunits to be added to assembling CcO, caused the most severe reduction in Cox1 synthesis. Deletion of the C-terminal 15 amino acids of Cox1 increased Cox1 synthesis in the presence of each of these mutations, except pet54. Our data suggest a novel activity of Pet54 required for normal synthesis of Cox1 that is independent of the Cox1 C-terminal end.

Dual functions of Mss51 couple synthesis of Cox1 to assembly of cytochrome c oxidase in Saccharomyces cerevisiae mitochondria

Functional interactions of the translational activator Mss51 with both the mitochondrially encoded COX1 mRNA 5'-untranslated region and with newly synthesized unassembled Cox1 protein suggest that it has a key role in coupling Cox1 synthesis with assembly of cytochrome c oxidase. Mss51 is present at levels that are near rate limiting for expression of a reporter gene inserted at COX1 in mitochondrial DNA, and a substantial fraction of Mss51 is associated with Cox1 protein in assembly intermediates. Thus, sequestration of Mss51 in assembly intermediates could limit Cox1 synthesis in wild type, and account for the reduced Cox1 synthesis caused by most yeast mutations that block assembly. Mss51 does not stably interact with newly synthesized Cox1 in a mutant lacking Cox14, suggesting that the failure of nuclear cox14 mutants to decrease Cox1 synthesis, despite their inability to assemble cytochrome c oxidase, is due to a failure to sequester Mss51. The physical interaction between Mss51 and Cox14 is dependent upon Cox1 synthesis, indicating dynamic assembly of early cytochrome c oxidase intermediates nucleated by Cox1. Regulation of COX1 mRNA translation by Mss51 seems to be an example of a homeostatic mechanism in which a positive effector of gene expression interacts with the product it regulates in a posttranslational assembly process.

Multiple defects in the respiratory chain lead to the repression of genes encoding components of the respiratory chain and TCA cycle enzymes

Respiratory complexes III, IV and V are formed by components of both nuclear and mitochondrial origin and are embedded in the inner mitochondrial membrane. Their assembly requires the auxiliary factor Oxa1, and the absence of this protein has severe consequences on these three major respiratory chain enzymes. We have studied, in the yeast Saccharomyces cerevisiae, the effect of the loss of Oxa1 function and of other respiratory defects on the expression of nuclear genes encoding components of the respiratory complexes and tricarboxylic acid cycle enzymes. We observed that the concomitant decrease in the level of two respiratory enzymes, complexes III and IV, led to their repression. These genes are known targets of the transcriptional activator complex Hap2/3/4/5 that plays a central role in the reprogramming of yeast metabolism when cells switch from a fermenting, glucose-repressed state to a respiring, derepressed state. We found that the Hap4 protein, the regulatory subunit of the transcriptional complex, was present at a lower level in the oxa1 mutants whereas no change in HAP4 transcript level was observed, suggesting a posttranscriptional modulation. In addition, an altered mitochondrial morphology was observed in mutants with decreased expression of Hap2/3/4/5 target genes. We suggest that the aberrant mitochondrial morphology, presumably caused by the severely decreased level of at least two respiratory enzymes, might be part of the signalling pathway linking the mitochondrial defect and Hap2/3/4/5.

Analysis of COX2 mutants reveals cytochrome oxidase subassemblies in yeast

Cytochrome oxidase catalyses the reduction of oxygen to water. The mitochondrial enzyme contains up to 13 subunits, 11 in yeast, of which three, Cox1p, Cox2p and Cox3p, are mitochondrially encoded. The assembly pathway of this complex is still poorly understood. Its study in yeast has been so far impeded by the rapid turnover of unassembled subunits of the enzyme. In the present study, immunoblot analysis of blue native gels of yeast wild-type and Cox2p mutants revealed five cytochrome oxidase complexes or subcomplexes: a, b, c, d and f; a is likely to be the fully assembled enzyme; b lacks Cox6ap; d contains Cox7p and/or Cox7ap; f represents unassembled Cox1p; and c, observed only in the Cox2p mutants, contains Cox1p, Cox3p, Cox5p and Cox6p and lacks the other subunits. The identification of these novel cytochrome oxidase subcomplexes should encourage the reexamination of other yeast mutants.

Mss51p promotes mitochondrial Cox1p synthesis and interacts with newly synthesized Cox1p

The post-transcriptional role of Mss51p in mitochondrial gene expression is of great interest since MSS51 mutations suppress the respiratory defect caused by shy1 mutations. SHY1 is a Saccharomyces cerevisiae homolog of human SURF1, which when mutated causes a cytochrome oxidase assembly defect. We found that MSS51 is required for expression of the mitochondrial reporter gene ARG8(m) when it is inserted at the COX1 locus, but not when it is at COX2 or COX3. Unlike the COX1 mRNA-specific translational activator PET309, MSS51 has at least two targets in COX1 mRNA. MSS51 acts in the untranslated regions of the COX1 mRNA, since it was required to synthesize Arg8p when ARG8(m) completely replaced the COX1 codons. MSS51 also acts on a target specified by the COX1 coding region, since it was required to translate either COX1 or COX1:: ARG8(m) coding sequences from an ectopic COX2 locus. Mss51p was found to interact physically with newly synthesized Cox1p, suggesting that it could coordinate Cox1p synthesis with insertion into the inner membrane or cytochrome oxidase assembly.

Mutations of cytochrome c oxidase subunits 1 and 3 in Saccharomyces cerevisiae: assembly defect and compensation

Eukaryotic cytochrome oxidases are composed of up to 13 subunits, of which three, subunits 1, 2 and 3, are mitochondrially encoded. In this study, yeast mutants were used to investigate the role of subunits 1 and 3 domains on the enzyme assembly. Mutation S203L in subunit 3 which abolished the respiratory growth, decreased cytochrome oxidase content, as measured by optical spectroscopy and immunodetection. Secondary mutations in subunits 1 and 3 restored (partly) the enzyme level. Two reversions reintroduced residues with a hydroxyl group at the primary mutation site (S203T) or in a subunit 3 transmembrane helix close to subunit 1 (G104S). These residues may be involved in hydrogen bonding which strengthen subunits 1-3 interaction. Two other reversions (A224V and Q137K) are located in P-side loops in subunit 1, which may be involved in the enzyme assembly. A mutation in residue A224 has been reported in a family presenting with encephalomyopathy. Surprisingly, the introduction of the 'human' mutation A224S and of a more drastic change A224F had no effect on the yeast enzyme. This might be explained by differences in local folding in the two enzymes.

Site-directed mutations in the mitochondrially encoded subunits I and III of yeast cytochrome oxidase

Since yeast is amenable to mitochondrial transformation, designed mutations can be introduced in the mitochondrially encoded subunits of the respiratory complexes. In the present work, six mutations have been introduced by the biolistic method into yeast (Saccharomyces cerevisiae) cytochrome oxidase subunits I and III. The effects of these mutations on respiratory growth competence, cytochrome oxidase activity and optical properties were then characterized. Firstly, the conserved glutamate Glu-243 in the D-channel of subunit I was replaced by an asparagine or an aspartate residue. The effects of the mutations showed that Glu-243, which is essential for proton movement in bacterial oxidases, is also required for the activity of the eukaryotic enzyme. Secondly, four mutations associated with human disease were introduced in yeast, allowing detailed analysis of their deleterious effects on cytochrome oxidase function: Met-273-->Thr, Ile-280-->Thr and Gly-317-->Ser, affecting residues located in or near the K-channel in subunit I, and a short in-frame deletion comprising residues Phe-102 to Phe-106 in subunit III (DeltaF102-F106). The subunit III mutation was highly deleterious and abolished enzyme assembly. The change Gly-317-->Ser had no effect on respiratory function. However, mutations Met-273-->Thr and Ile-280-->Thr were mildly deleterious, decreased cytochrome oxidase activity and slightly perturbed the properties of the binuclear centre.

Second-site reversion analysis is not a reliable method to determine distances in membrane proteins: an assessment using mutations in yeast cytochrome c oxidase subunits I and II

We have examined deficiency mutations and reversions in subunits I and II of yeast cytochrome c oxidase in order to test the reliability of second-site reversion analysis in prediction of tertiary structure of a membrane protein complex. It appears that the method can not provide information on distance between residues, since reversions can be up to 30 A from the primary mutations. However, the reversions are not randomly located in the structure but reveal regions essential for assembly or functional units.

Effects of mutation of residue I67 on redox-linked protonation processes in yeast cytochrome c oxidase

We describe effects of a mutation, Ile-67-->Asn, in subunit I of yeast cytochrome c oxidase on redox-linked protonation processes within the protein. The mutation lowers the midpoint potential of haem a and weakens its pH dependency, but has little effect on the potential of haem a3. The residue is close to a conserved glutamate (Glu-243) in the crystal structure. We propose that protonation of Glu-243 is redox-linked to haem a, that Asn-167 perturbs its pK and that redox-linked protonation in this location is essential for the catalytic reactions of the binuclear centre. These proposals are discussed in terms of a 'glutamate trap' mechanism for proton translocation in the haem/copper oxidases.

Structural and functional analysis of deficient mutants in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae

Four point mutations in subunit I of cytochrome c oxidase from Saccharomyces cerevisiae that had been selected for respiratory incompetence but still contained spectrally detectable haem aa3 were analysed. The isolated mutant enzymes exhibited minor band shifts in their optical spectra and contained all eleven subunits. However, steady state activities were only a few percent compared to wild type enzyme. Using a comprehensive experimental approach, we first checked the integrity of the enzyme preparations and then identified the specific functional defect. The results are discussed using information from the recently solved structures of cytochrome c oxidase at 2.8 A. Mutation 167N is positioned between haem a and a conserved glutamate residue (E243). It caused a distortion of the EPR signal of haem a and shifted its midpoint potential by 54 mV to the negative. The high-resolution structure suggests that the primary reason for the low activity of the mutant enzyme could be that asparagine in position 67 might form a stable hydrogen bond to E243, which is part of a proposed proton channel. Cytochrome c oxidase isolated from mutant T316K did not meet our criteria for homogeneity and was therefore omitted from further analysis. Mutants G352V and V380M exhibited an impairment of electron transfer from haem a to a3 and ligand binding to the binuclear centre was affected. In mutant V380M also the midpoint potential of CuB was shifted by 65 mV to the positive. The results indicated for these two mutants changes primarily associated with the binuclear centre, possibly associated with an interference in the routes and/or sites of protonation which are required for stable formation of the catalytic intermediates. This interpretation is discussed in the light of the high resolution structure.

The topology of CuA in relation to the other metal centres in cytochrome-c oxidase of Saccharomyces cerevisiae as determined by analysis of second-site reversions

Second-site revertants were selected from a respiratory-deficient mutant carrying the mutation D369N located in a loop between helices IX and X close to H376 and H378, the proposed ligands of haem a3 and haem a, respectively. A reversion was observed in subunit II, in the vicinity of the CuA ligands. This same reversion compensates the subunit I deficiency mutation, S140L, assumed to be near H62, the second putative histidine ligand to haem a. These data enable us to propose a three-dimensional topology in which CuA in subunit II is located on top of the Positive-side of subunit I and in proximity to all three of its metal centres.

Identification of essential amino acids within the proposed CuA binding site in subunit II of Cytochrome c oxidase

To explore the nature of proposed ligands to the CuA center in cytochrome c oxidase, site-directed mutagenesis has been initiated in subunit II of the enzyme. Mutations were introduced into the mitochondrial gene from the yeast Saccharomyces cerevisiae by high velocity microprojectile bombardment. A variety of single amino acid substitutions at each of the proposed cysteine and histidine ligands (His-161, Cys-196, Cys-200, and His-204 in the bovine numbering scheme), as well as at the conserved Met-207, all result in yeast which fails to grow on ethanol/glycerol medium. Similarly, all possible paired exchange Cys,His and Cys,Met mutants show the same phenotype. Furthermore, protein stability is severely reduced as evidenced by both the absence of an absorbance maximum at 600 nm in the spectra of mutant cells and the underaccumulation of subunit II, as observed by immunolabeling of mitochondrial extracts. In the same area of the protein, a variety of amino acid substitutions at one of the carboxylates previously implicated in binding cytochrome c, Glu-198, allow (reduced) growth on ethanol/glycerol medium, with normal intracellular levels of protein. These results suggest that a precise folding environment of the CuA site within subunit II is essential for assembly or stable accumulation of cytochrome c oxidase in yeast.

Kinetic properties and ligand binding of the eleven-subunit cytochrome-c oxidase from Saccharomyces cerevisiae isolated with a novel large-scale purification method

A novel, large-scale method for the purification of cytochrome-c oxidase from the yeast Saccharomyces cerevisiae is described. The isolation procedure gave highly pure and active enzyme at high yields. The purified enzyme exhibited a heme a/protein ratio of 9.1 mmol/mg and revealed twelve protein bands after Tricine/SDS/PAGE. N-terminal sequencing showed that eleven of the corresponding proteins were identical to those recently described by Taanman and Capaldi [Taanman, J.-W. & Capaldi, R.A. (1992) J. Biol. Chem. 267, 22,481-22,485]. 15 of the N-terminal residues of the 12th band were identical to subunit VIII indicating that this band represents a dimer of subunit VIII (M(r) 5364). We conclude that subunit XII postulated by Taanman and Capaldi is the subunit VIII dimer and that cytochrome-c oxidase contains eleven rather than twelve subunits. We obtained the complete sequence of subunit VIa by Edman degradation. The protein contains more than 25% of charged amino acids and hydropathy analysis predicts one membrane-spanning helix. The purified enzyme had a turnover number of 1500 s-1 and the ionic-strength dependence of the Km value for cytochrome-c was similar to that described for other preparations of cytochrome-c oxidase. This was also true for the cyanide-binding characteristics of the preparation. When the enzyme was isolated in the presence of chloride, more than 90% of the preparation showed fast cyanide-binding kinetics and was resistant to formate incubation, indicating that chloride was bound to the binuclear center. When the enzyme was isolated in the absence of chloride, approximately 70% of the preparation was in the fast form. This high content of fast enzyme was also reflected in the characteristics of optical and EPR spectra for cytochrome-c oxidase purified with our method.

Flash photolysis of the carbon monoxide compounds of wild-type and mutant variants of cytochrome bo from Escherichia coli

The carbon monoxide compounds of the fully reduced and mixed valence forms of cytochrome bo from Escherichia coli were laser photolysed under anaerobic conditions at room temperature. The carbon monoxide recombined with characteristic rate constants of 50 s-1 or 35 s-1 in the fully reduced and mixed valence forms, respectively. Rates of CO recombination with the fully reduced enzyme were examined in a variety of mutant forms of cytochrome bo, produced by site-directed mutagenesis. A method was developed to deconvolute cytochromes bo and bd, leading to some reassessment of histidine ligands to the metals. Significant changes in the rate constant of recombination of carbon monoxide occurred in many of these mutants and these results could be rationalised generally in terms of our current working model of the folding structure of subunit I. In the mixed valence form of the enzyme the transient photolysis spectra in the visible region are consistent with a rapid electron redistribution from the binuclear centre to the low-spin haem. This electron transfer is biphasic, with rate constants of around 10(5) and 8000 s-1. The process was also examined in the His-333-Leu mutant, in which a putative histidine ligand to CuB is replaced by leucine, and which results in the loss of the CuB. It appeared that rapid haem-haem electron transfer could still occur. The observation that CuB is apparently not required for rapid haem-haem electron transfer is consistent with the recently proposed model in which the two haems are positioned on opposite sides of transmembrane helix X in subunit I of the oxidase.

Insight into the interactions between subunits I and II of the cytochrome c oxidase of the yeast Saccharomyces cerevisiae by means of extragenic complementation

In yeast, revertants were selected from two respiratory deficient mutants carrying mutations in the catalytic subunits of cytochrome oxidase. From a mutant carrying a double mutation in the vicinity of the copper binding pocket in subunit II, two genetically independent revertants were obtained in which the same extragenic reversion mutation was observed, A147V, in the putative helix 4 of subunit I. A comparison with revertants derived from the second deficient mutant, carrying the deficiency mutation, S140L, in the loop 3-4 of subunit I, provides additional data in favour of an interaction between helix 4 of subunit I and subunit II.

Random mutant generation and its utility in uncovering structural and functional features of cytochrome b in Saccharomyces cerevisiae

The generation of random mutations in the mitochondrial cytochrome b gene of Saccharomyces cerevisiae has been used as a most fruitful means of identifying subregions that play a key role in the bc1 complex mechanism, best explained by the protonmotive Q cycle originally proposed by Peter Mitchell. Selection for center i and center o inhibitor resistance mutants, in particular, has yielded much information. The combined approaches of genetics and structural predictions have led to a two-dimensional folding model for cytochrome b that is most compatible with current knowledge of the protonmotive Q cycle. A three-dimensional model is emerging from studies of distant reversions of deficient mutants. Finally, interactions between cytochrome b and the other subunits of the bc1 complex, such as the iron-sulfur protein, can be affected by a single amino acid change.

Rapid screening of cytochromes of respiratory mutants of Saccharomyces cerevisiae. Application to the selection of strains containing novel forms of cytochrome-c oxidase

A technique has been developed for the direct analysis by visible spectrophotometry of yeast spots growing on agar plates. This allows rapid semi-quantitative estimations of cytochromes c, b and oxidase and permits the identification of strains with impaired respiratory electron flow. Results of screening of 105 mutants are presented. There appears to be a correlation between the exonic location of the mutation in COX1 of oxidase and the level of optically detectable enzyme. Mutations in cytochrome b of the bc1 complex also affect the level of expression of cytochrome oxidase and can cause either an increased or decreased level of expression of oxidase relative to the wild-type strain. Twelve strains selected by the rapid level-1 screening were grown as lawns on sections of an agar plate and resuspended for a second level of screening. Quantitative estimates have been made of the concentrations of cytochromes, the turnover number of cytochrome oxidase and the kinetics of recombination of carbon monoxide with oxidase after flash photolysis. This confirmed the validity of the rapid screening procedure, and we have identified several strains which contain high levels of a mutant form of cytochrome oxidase with properties worthy of further investigation.