Purification and characterization of the inactive Ca2+, Mg2+-activated adenosine triphosphatase of the unc A- mutant Escherichia coli AN120

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

Contact-dependent growth inhibition causes reversible metabolic downregulation in Escherichia coli

Contact-dependent growth inhibition (CDI) is a mechanism identified in Escherichia coli by which bacteria expressing two-partner secretion proteins encoded by cdiA and cdiB bind to BamA in the outer membranes of target cells and inhibit their growth. A third gene in the cluster, cdiI, encodes a small protein that is necessary and sufficient to confer immunity to CDI, thereby preventing cells expressing the cdiBA genes from inhibiting their own growth. In this study, the cdiI gene was placed under araBAD promoter control to modulate levels of the immunity protein and thereby induce CDI by removal of arabinose. This CDI autoinhibition system was used for metabolic analyses of a single population of E. coli cells undergoing CDI. Contact-inhibited cells showed altered cell morphology, including the presence of filaments. Notably, CDI was reversible, as evidenced by resumption of cell growth and normal cellular morphology following induction of the CdiI immunity protein. Recovery of cells from CDI also required an energy source. Cells undergoing CDI showed a significant, reversible downregulation of metabolic parameters, including aerobic respiration, proton motive force (Deltap), and steady-state ATP levels. It is unclear whether the decrease in respiration and/or Deltap is directly involved in growth inhibition, but a role for ATP in the CDI mechanism was ruled out using an atp mutant. Consistent with the observed decrease in Deltap, the phage shock response was induced in cells undergoing CDI but not in recovering cells, based on analysis of levels of pspA mRNA.

Alterations of cellular physiology in Escherichia coli in response to oxidative phosphorylation impaired by defective F1-ATPase

The physiological changes in an F1-ATPase-defective mutant of Escherichia coli W1485 growing in a glucose-limited chemostat included a decreased growth yield (60%) and increased specific rates of both glucose consumption (168%) and respiration (171%). Flux analysis revealed that the mutant showed approximately twice as much flow in glycolysis but only an 18% increase in the tricarboxylic acid (TCA) cycle, owing to the excretion of acetate, where most of the increased glycolytic flux was directed. Genetic and biochemical analyses of the mutant revealed the downregulation of many TCA cycle enzymes, including citrate synthase, and the upregulation of the pyruvate dehydrogenase complex in both transcription and enzyme activities. These changes seemed to contribute to acetate excretion in the mutant. No transcriptional changes were observed in the glycolytic enzymes, despite the enhanced glycolysis. The most significant alterations were found in the respiratory-chain components. The total activity of NADH dehydrogenases (NDHs) and terminal oxidases increased about twofold in the mutant, which accounted for its higher respiration rate. These changes arose primarily from the increased (3.7-fold) enzyme activity of NDH-2 and an increased amount of cytochrome bd in the mutant. Transcriptional upregulation appeared to be involved in these phenomena. As NDH-2 cannot generate an electrochemical gradient of protons and as cytochrome bd is inferior to cytochrome bo3 in this ability, the mutant was able to recycle NADH at a higher rate than the parent and avoid generating an excess proton-motive force. We discuss the physiological benefits of the alterations in the mutant.

Changes in the adenine nucleotide and inorganic phosphate content of Escherichia coli F1-ATPase during ATP synthesis in dimethyl sulphoxide

Escherichia coli F1-ATPase contained 2.9 +/- 0.1 mol of adenine nucleotide and 3.1 +/- 0.3 mol of Pi/mol of enzyme. After preincubation with ATP, the nucleotide and phosphate contents were 5.6 and 6.0 +/- 0.5 mol/mol of enzyme respectively. The F1-ATPase was induced to synthesize ATP in the presence of 30% (v/v) dimethyl sulphoxide (Me2SO). The ATP originated from endogenous bound ADP. The bound adenine nucleotide and Pi contents of the enzyme during the time course of ATP synthesis were investigated by using F1-ATPase which had been preincubated with ATP. We show that the process of ATP synthesis in Me2SO involves (i) an initial rapid loss of nucleotide from the enzyme, the process being facilitated by exogenous Pi, (ii) a rapid loss of Pi from the enzyme, at least in the absence of exogenous Pi, (iii) re-binding of a portion of the lost nucleotide, and (iv) synthesis of ATP from bound ADP and exogenous Pi. It is proposed that transfer of the F1-ATPase to the Me2SO medium induces a change in the conformation of the enzyme to a form favouring ATP synthesis.

A comparison of an ATPase from the archaebacterium Halobacterium saccharovorum with the F1 moiety from the Escherichia coli ATP synthase

A purified ATPase associated with membranes from Halobacterium saccharovorum was compared with the F1 moiety from the Escherichia coli ATP synthase. The halobacterial enzyme was composed of two major (I and II) and two minor subunits (III and IV), whose molecular masses were 87 kDa, 60 kDa, 29 kDa and 20 kDa, respectively. The isoelectric points of these subunits ranged from 4.1 to 4.8, which in the case of the subunits I and II was consistent with the presence of an excess of acidic amino acids (20-22 mol/100 mol). Peptide mapping of subunits I and II denatured with sodium dodecyl sulfate showed no relationship between the primary structures of the individual halobacterial subunits or similarities to the subunits of the F1 ATPase from E. coli. Trypsin inactivation of the halobacterial ATPase was accompanied by the partial degradation of the major subunits. This observation, taken in conjunction with molecular masses of the subunits and the native enzyme, was consistent with the previously proposed stoichiometry of 2:2:1:1. These results suggest that H. saccharovorum, and possibly, halobacteria in general, possess an ATPase which is unlike the ubiquitous F0F1 ATP synthase.

Ligand-induced conformational changes in the Escherichia coli F1 adenosine triphosphatase probed by trypsin digestion

Digestion of the F1-ATPase of Escherichia coli with trypsin stimulated ATP hydrolytic activity and removed the delta and epsilon subunits of the enzyme. A species represented by the formula alpha 1(3) beta 1(3) gamma 1, where alpha 1, beta 1 and gamma 1 are forms of the native alpha, beta and gamma subunits which have been attacked by trypsin, was formed by trypsin digestion in the presence of ATP. In the presence of ATP and MgCl2, conversion of gamma to gamma 1 was retarded and the enzyme retained the epsilon subunit. These results imply that binding of ATP to the beta subunits alters the conformation of ECF1 to increase the accessibility of the gamma subunit to trypsin. The likely trypsin cleavage sites in the alpha, beta and gamma subunits are discussed. ECF1 from the alpha subunit-defective mutant uncA401, or after treatment with N,N'-dicyclohexylcarbodiimide or 4-chloro-7-nitrobenzofurazan, was present in a conformation in which the gamma subunit was readily accessible to trypsin and could not be protected by the presence of ATP and MgCl2. In a similar manner to native E. coli F1-ATPase, the hydrolytic activity of the trypsin-digested enzyme was stimulated by the detergent lauryldimethylamine N-oxide. Since the digested enzyme lacked the epsilon subunit, a putative inhibitor of hydrolytic activity, a mechanism for the stimulation which involves loss or movement of this subunit is untenable.

Effect of disulfide cross-linking between alpha and delta subunits on the properties of the F1 adenosine triphosphatase of Escherichia coli

Under very mild oxidizing conditions the delta subunit of the F1-ATPase of Escherichia coli can be crosslinked by a disulfide linkage to one of the alpha subunits of the enzyme. The cross-linked ATPase resembles the native enzyme in the following properties: specific activity; activation by lauryldimethylamine N-oxide (LDAO); binding of aurovertin D and ADP; cross-linking products with 3,3'-dithiobis(succinimidyl propionate); binding to ATPase-stripped everted membrane vesicles and the N,N'-dicyclohexylcarbodiimide sensitivity of the rebound enzyme. However, the rebound crosslinked ATPase differed from the native enzyme in lacking the ability to restore NADH oxidation - and ATP hydrolysis-dependent quenching of the fluorescence of quinacrine to ATPase-stripped membrane vesicles. It is proposed that the delta subunit is involved in the proton pathway of the ATPase, and that this pathway is affected in the alpha delta-cross-linked enzyme. The mechanism for activation of the ATPase by LDAO was examined. Evidence against the proposal of Lötscher, H.-R., De Jong, C. and Capaldi, R.A. (Biochemistry (1984) 23, 4140-4143) that activation involves displacement of the epsilon subunit from an active site on a beta subunit was obtained.

Conformational interactions between alpha and beta subunits in the F1 ATPase of Escherichia coli as shown by chemical modification of uncA401 and uncD412 mutant enzymes

In contrast to wild-type F1 adenosine triphosphatase, the beta subunits of soluble ATPase from Escherichia coli mutant strains AN120 (uncA401) and AN939 (uncD412) were not labeled by the fluorescent thiol-specific reagents 5-iodoacetamidofluorescein, 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid or 4-[N-(iodoacetoxy)ethyl-N-methyl]amino-7-nitrobenzo-2-oxa-1,3-diazole. The mutation in the alpha subunit (uncA401) of F1 ATPase thus influences the accessibility of the single cysteinyl residue in the beta subunit. Following reaction of ATPase with 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole or N,N'-dicyclohexylcarbodiimide, the alpha and beta subunits of the uncA401, but not of the uncD412 mutant F1 ATPase were intensely labeled by a fluorescent thiol reagent. The mutation in the beta subunit (uncD412) thus influences the accessibility of the cysteinyl residues in the alpha subunit. In other work [Stan-Lotter, H. and Bragg, P.D. (1986) Arch. Biochem. Biophys. 248] we have shown that 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole and 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid react with a different beta subunit from that labeled by N,N'-dicyclohexylcarbodiimide. This asymmetry with respect to modification by 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole and N,N'-dicyclohexylcarbodiimide was seen in both mutant enzymes. In addition, the modification of one beta subunit of the uncA401 F1 ATPase induced the previously unreactive sulfhydryl group of another beta subunit to react with 2-(4'-iodoacetamidoanilino-naphthalene-6-sulfonic acid. These results provide evidence for at least three types of conformational interactions of the major subunits of F1 ATPase: from alpha to beta, from beta to alpha, and from beta to beta. As in wild-type ATPase, labeling of membrane-bound unc mutant ATPase by a fluorescent thiol reagent modified the alpha subunits. This suggests that a conformational change of yet a different type occurs when the enzyme binds to the membrane.

N,N'-dicyclohexylcarbodiimide and 4-chloro-7-nitrobenzofurazan bind to different beta subunits of the F1 ATPase of Escherichia coli

The fluorescent thiol reagent 2-(4'-iodoacetamidoanilino)naphthalene-6-sulfonic acid (IAANS) labels the gamma, delta, and one of the three beta subunits of the F1 ATPase from Escherichia coli (ECF1). This is the same beta subunit which incorporates 4-chloro-7-nitrobenzofurazan (Nbf) [H. Stan-Lotter and P. D. Bragg (1986) Eur. J. Biochem. 154, 321-327]. After inactivation of ECF1 with N,N'-dicyclohexylcarbodiimide (DCCD), IAANS labels in addition to the beta, gamma, and delta subunits also the alpha subunit. This suggests a conformational change of ECF1 upon binding of DCCD. The beta subunit which incorporates DCCD does does not bind IAANS. Likewise, IAANS-modified ECF1 does not incorporate DCCD into the same beta subunit. It is concluded that DCCD and Nbf bind to different beta subunits. Since neither of these reagents binds to that beta subunit which can be crosslinked to to the epsilon subunit by 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide, these data show that there is a difference in the chemical reactivity of each of the three beta subunits of ECF1, despite their identical primary structures. This suggests that there is an asymmetry in the F1 molecule.

Characterisation of phosphate binding to mitochondrial and bacterial membrane-bound ATP synthase by studies of inhibition with 4-chloro-7-nitrobenzofurazan

The effect of phosphate on the inhibition by 4-chloro-7-nitrobenzofurazan of the ATPase activity of the proton-translocating ATP synthase in heart submitochondrial particles was investigated. Binding of phosphate protected strongly against the inhibition. A dissociation constant of 0.2 mM was determined for the enzyme X Pi complex and shown to be independent of pH in the range 7.0-8.0. The protective effect of phosphate was mimicked by arsenate but not by sulphate or malonate. Similar results were obtained for the enzyme from Paracoccus denitrificans. 2,4-Dinitrophenol enhanced phosphate binding to the mitochondrial enzyme since the protective effect of phosphate was increased. The data are compatible with protection arising from binding of phosphate to a catalytic site.

Thiol modification as a probe of conformational forms of the F1 ATPase of Escherichia coli and of the structural asymmetry of its beta subunits

The sulfhydryl groups of soluble and membrane-bound F1 adenosine triphosphatase of Escherichia coli were modified by reaction with the fluorescent thiol reagents 5-iodoacetamidofluorescein, 2-[(4'-iodoacetamido)anilino]naphthalene-6-sulfonic acid 4-[N-(iodoacetoxy)ethyl-N-methyl]amino-7-nitrobenzo-2-oxa-1,3-d iaz ole and 2-[(4'-maleimidyl)anilino]naphthalene-6-sulfonic acid. Whereas gamma and delta subunits were always labeled by these reagents, the beta subunit reacted preferentially in the soluble enzyme, and the alpha subunit in the membrane-bound enzyme. This suggests that the soluble enzyme undergoes a conformational change on binding to the membrane. The three beta subunits of the soluble ATPase did not react with chemical reagents in a similar manner. One beta subunit was cross-linked to the epsilon subunit on treatment of the ATPase with 1-ethyl-3-[3-(dimethyl-amino)propyl]carbodiimide, as observed previously by Lötscher et al. [Biochemistry (1984) 23, 4134-4140]. A second beta subunit, which did not cross-link to the epsilon subunit, was modified preferentially by the fluorescent thiol reagents and by 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole. The third beta subunit was less chemically reactive than the others. Both alpha and beta subunits of the soluble 4-chloro-7-nitrobenzo-2-oxa-1,3-diazole-modified enzyme were labeled by the fluorescent thiol reagents. Thus, the modified enzyme, which is inactive, probably has a different conformation from the native soluble ATPase.

Catalytic properties of the Escherichia coli proton adenosinetriphosphatase: evidence that nucleotide bound at noncatalytic sites is not involved in regulation of oxidative phosphorylation

Nucleotide-depleted F1-ATPase from Escherichia coli was reconstituted with F1-depleted membranes and shown to catalyze high rates of oxidative phosphorylation of ADP and GDP. Adenine nucleotide became bound to the nonexchangeable nucleotide sites on membrane-bound F1 during ATP synthesis, but binding of guanine nucleotides to nonexchangeable sites during GTP synthesis was not detectable. It was possible to reload the nonexchangeable sites on nucleotide-depleted F1 with radioactive adenine nucleotide prior to membrane reconstitution. The radioactive adenine nucleotide did not exchange significantly during oxidative phosphorylation of ADP or GDP. The amount of nonexchangeable adenine nucleotide found in membrane-bound F1 was the same when the nonexchangeable sites were reloaded either prior to membrane reconstitution of the F1 or after membrane reconstitution with nucleotide-free F1 followed by a burst of oxidative phosphorylation of ADP. The results showed that occupation of the nonexchangeable sites on F1 by tightly bound nucleotide is not required for oxidative phosphorylation of GDP (a physiological activity of F1 in the bacterial cell). Also, the results confirm directly that the adenine-specific nonexchangeable sites on F1 are noncatalytic sites. Using this experimental approach, it was possible to look for a regulatory effect of the nonexchangeable nucleotide on oxidative phosphorylation. Nucleotide-depleted F1 was first reloaded with (i) ATP, (ii) ADP, (iii) 5'-adenylyl imidodiphosphate, or (iv) zero nucleotide, and was then reconstituted with F1-depleted membranes. The reconstituted membranes were compared in respect to rates of oxidative phosphorylation of GDP and Km values of GDP and Pi. No regulatory role for the nonexchangeable nucleotide was evident.(ABSTRACT TRUNCATED AT 250 WORDS)

Tyrosine-311 of a beta chain is the essential residue specifically modified by 4-chloro-7-nitrobenzofurazan in bovine heart mitochondrial ATPase

A peptide containing an essential tyrosine residue, modified with the nitrobenzofurazan group, has been purified from bovine heart mitochondrial ATPase. The composition of the peptide indicates that this tyrosine is residue 311 in the sequence of a beta chain. The problem of the instability of the tyrosyl-nitrobenzofurazan bond was overcome by working throughout at relatively acidic pH and using pepsin digestion of the enzyme.

Photosynthetic ATPases: purification, properties, subunit isolation and function

Photosynthetic coupling factor ATPases (F1-ATPases) generally censist of five subunits named α, β, γ, δ and ε in order of decreasing apparent molecular weight. The isolated enzyme has a molecular weight of between 390,000 to 400,000, with the five subunits probably occurring in a 3:3:1:1:1 ratio. Some photosynthetic F1 ATPases are inactive as isolated and require treatment with protease, heat or detergent in order to elicit ATPase activity. This activity is sensitive to inhibition by free divalent cations and appears to be more specific for Ca(2+) vs. Mg(2+) as the metal ion substrate chelate. This preference for Ca(2+) can be explained by the higher inhibition constant for inhibition of ATPase activity by free Ca(2+). Methods for the assay of a Mg-dependent ATPase activity have recently been described. These depend on the presence of organic solvents or detergents in the reaction mixture for assay. The molecular mechanism behind the expression of either the Ca- or Mg-ATPase activities is unknown. F1-ATPases function to couple proton efflux from thylakoid membranes or chromatophores to ATP synthesis. The isolated enzyme may thus also be assayed for the reconstitution of 'coupling activity' to membranes depleted of coupling factor 1.The functions of the five subunits in the complex have been deduced from the results of chemical modification and reconstitution studies. The δ subunit is required for the functional binding of the F1 to the F0. The active site is probably contained in the β (and α) subunit(s). The proposed functions for the γ and ε subunits are, however, still matters of controversy. Coupling factors from a wide variety of species including bacteria, algae, C3 and C4 plants, appear to be immunologically related. The β subunits are the most strongly related, although the α and γ subunits also show significant immunological cross-reactivity. DNA sequence analyses of the genes for the β subunit of CF1 have indicated that the primary sequence of this polypeptide is highly conserved. The genes for the polypeptides of CF1 appear to be located in two cellular compartments. The α, β and ε subunits are coded for on chloroplast DNA, whereas the γ and δ subunits are probably nuclear encoded. Experiments involving protein synthesis by isolated chloroplasts or protein synthesis in the presence of inhibitors specific for one or the other set of ribosomes in the cell suggest the existence of pools of unassembled CF1 subunits. These pools, if they do exist in vivo, probably make up no greater than 1% of the total CF1 content of the cell.

Identification of an essential beta chain lysine residue from bovine heart mitochondrial ATPase specifically modified with nitrobenzofurazan

A tetrapetide containing an essential lysine residue chemically modified with the nitrobenzofurazan group has been purified from bovine heart mitochondrial ATPase. The composition of the peptide indicates that this lysine is residue 401 in the sequence of a beta chain. The modification was achieved by incubation at pH 9 of ATPase that had been previously labelled on a single essential tyrosine residue by reaction of the enzyme with 4-chloro-7-nitrobenzofurazan. The specific transfer of the nitrobenzofurazan group from the tyrosine residue to a particular lysine residue is consistent with the previously demonstrated intramolecular character of this transfer reaction.

Loss of protection by nucleotides against proteolysis and thiol modification in the isolated alpha-subunit from F1 ATPase of Escherichia coli mutant uncA401

Binding of nucleotides to the high-affinity site of the isolated alpha subunit of normal Escherichia coli F1 adenosine triphosphatase (ATPase) results in partial protection against digestion by trypsin [Senda, Kanazawa, Tsuchiya & Futai (1983) Arch. Biochem. Biophys. 220, 398-440]. In contrast, the isolated alpha subunit from the defective ATPase of the E. coli uncA401 mutant (strain AN120) is cleaved by trypsin to peptides of less than 8000 Da in the presence of ADP or ATP (2.5 microM-110 mM). The nucleotide-dependent accessibility of thiol groups of the isolated alpha subunit was also studied. Two out of four thiol groups of the alpha subunit from normal ATPase are labelled by fluorescent maleimides or iodoacetates, but in the presence of ADP or ATP (0.14-1.2 mM), reaction of thiol groups with these labels is almost absent. Mutant alpha subunit, however, is labelled by these reagents at all four thiol groups in the presence or absence of ADP or ATP (1 mM). These results suggest that the mutation in the ATPase of strain AN120 leads either to the loss of the high-affinity nucleotide-binding site or affects transmission of allosteric changes that occur on binding of nucleotide to the isolated alpha subunit.

Specificity of the proton adenosinetriphosphatase of Escherichia coli for adenine, guanine, and inosine nucleotides in catalysis and binding

Specificity of the Escherichia coli proton ATPase for adenine, guanine, and inosine nucleotides in catalysis and binding was studied. MgADP, CaADP, MgGDP, and MgIDP were each good substrates for oxidative phosphorylation. The corresponding triphosphates were each substrates for hydrolysis and proton pumping. At 1 mM concentration, MgATP, MgGTP, and MgITP drove proton pumping with equal efficiency. At 0.1 mM concentration, MgATP was 4-fold more efficient than MgITP or MgGTP. Nucleotide-depleted soluble F1 could rebind to F1-depleted membranes and block proton conductivity through F0; rebound nucleotide-depleted F1 catalyzed pH gradient formation with MgATP, MgGTP, or MgITP. This showed that the nonexchangeable nucleotide sites on F1 need not be occupied by adenine nucleotide for proton pumping to occur. It was further shown that no nucleotide was tightly bound in the nonexchangeable sites of F1 during proton pumping driven by MgGTP in these reconstituted membranes, whereas adenine nucleotide was tightly bound when MgATP was the substrate. Nucleotide-depleted soluble F1 bound maximally 5.9 ATP, 3.2 GTP, and 3.6 ITP of which half the ATP and almost all of the GTP and ITP exchanged over a period of 30-240 min with medium ADP or ATP. Also, half of the bound ATP exchanged with medium GTP or ITP. These data showed that inosine and guanine nucleotides do not bind to soluble F1 in nonexchangeable fashion, in contrast to adenine nucleotides. Purified alpha-subunit from F1 bound ATP at a single site but showed no binding of GTP nor ITP, supporting previous suggestions that the non-exchangeable sites in intact F1 are on alpha-subunits.

The nature of the reaction of an essential tyrosine residue of bovine heart mitochondrial ATPase with 4-chloro-7-nitrobenzofurazan and related compounds

Bovine heart mitochondrial ATPase is inhibited after covalent modification with 4-chloro-7-nitrobenzofuroxan. The kinetics of the reaction are indistinguishable from those for the reaction of an essential tyrosine residue of the ATPase with 4-chloro-7-nitrobenzofurazan that have been described previously [Ferguson et al. (1975) Eur. J. Biochem. 54, 117-126]. 4-Fluoro-7-nitrobenzofurazan inhibits the ATPase with a pseudo-first-order rate constant that is tenfold greater than that for 4-chloro-7-nitrobenzofurazan. These data indicate that the rate-limiting step for reaction of the enzyme with these reagents is formation of a Meisenheimer complex at the C-4 position and that the modified tyrosine is probably on the surface of the protein. No evidence was found for more complex patterns of reactivity of 4-chloro-7-nitrobenzofurazan and its analogues. Both ammonium 4-chloro-7-sulphobenzofurazan and ammonium 4-fluoro-7-sulphobenzofurazan fail to react with the ATPase. The utility of these reagents as alternatives to the nitro derivatives may be limited owing to their slow reaction rates. After modification on tyrosine by 4-chloro-7-nitrobenzofurazan, the nitrobenzofurazan group can be transferred by an intramolecular process to lysine [Ferguson et al. (1975) Eur. J. Biochem. 54, 127-133]. ATPase with the lysine thus modified is shown to be reactive towards 4-chloro-7-nitrobenzofurazan in a manner indistinguishable from the native enzyme. This indicates that the intramolecular transfer occurs at sufficient distance to avoid steric hindrance to the second reaction, and that the lysine does not participate in a neighbouring group effect to enhance the reactivity of the tyrosine.