Biochemical genetics of further chlorate resistant, molybdenum cofactor defective, conditional-lethal mutants of barley

Summary Three plants, R9201 and R11301 (from cv. Maris Mink) and R12202 (from cv. Golden Promise), were selected by screening M₂ populations of barley (Hordeum vulgare L.) seedlings (mutagenised with azide in the M₁) for resistance to 10 mM potassium chlorate. Selections R9201 and R11301 were crossed with the wild-type cv. Maris Mink and analysis of the F₂ progeny showed that one quarter lacked shoot nitrate reductase activity. These F₂ plants also withered and died in the continuous presence of nitrate as sole nitrogen source. Loss of nitrate reductase activity and withering and death were due in each case to a recessive mutation in a single nuclear gene. All F1 progeny derived from selfing selection R12202 lacked shoot nitrate reductase activity and also withered and subsequently died when maintained in the continuous presence of nitrate as sole nitrogen source. All homozygous mutant plants lacked not only shoot nitrate reductase activity but also shoot xanthine dehydrogenase activity. The plants took up nitrate, and possessed wild-type or higher levels of shoot nitrite reductase activity and NADH-cytochrome c reductase activity when treated with nitrate for 18 h. We conclude that loss of shoot nitrate reductase activity, xanthine dehydrogenase activity and withering and death, in the three mutants R9201, R11301 and R12202 is due to a mutation affecting the formation of a functional molybdenum cofactor. The mutants possessed wild-type levels of molybdenum and growth in the presence of unphysiologically high levels of molybdate did not restore shoot nitrate reductase or xanthine dehydrogenase activity. The shoot molybdenum cofactor of R9201 and of R12202 is unable to reconstitute NADPH nitrate reductase activity from extracts of the Neurospora crassa nit-1 mutant and dimerise the nitrate reductase subunits present in the respective barley mutant. The shoot molybdenum cofactor of R11301 is able to effect dimerisation of the R11301 nitrate reductase subunits and can reconstitute NADPH-nitrate reductase activity up to 40% of the wild-type molybdenum cofactor levels. The molybdenum cofactor of the roots of R9201 and R11301 is also defective. Genetic analysis demonstrated that R9201, but not R11301, is allelic to R9401 and Az34 (nar-2a), two mutants previously shown to be defective in synthesis of molybdenum cofactor. The mutations in R9401 and R9201 gave partial complementation of the nar-2a gene such that heterozygotes had higher levels of extractable nitrate reductase activity than the homozygous mutants.We conclude that: (a) the nar-2 gene locus encodes a step in molybdopterin biosynthesis; (b) the mutant R11301 represents a further locus involved in the synthesis of a functional molybdenum cofactor; (c) mutant Rl2202 is also defective in molybdopterin biosynthesis; and (d) the nar-2 gene locus and the gene locus defined by R11301 govern molybdenum cofactor biosynthesis in both shoot and root.     

Characterization of a new type of molybdenum cofactor-mutant in cell cultures of Nicotiana tabacum

Summary Four allelic putative cnx (molybdenum-cofactor defective) cell lines (O42, P12, P31 and P47) of Nicotiana tabacum var. Xanthi, biochemically and genetically distinct from N. tabacum var. Gatersleben cnxA mutants, were examined further. Their molybdenum-cofactor could efficiently reconstitute NADPH-nitrate reductase activity from Neurospora crassa mutant nit-1 extract only in the presence of exogenous molybdenum unlike that of the wild-type cofactor which could reconstitute NADPH-nitrate reductase activity in either the absence or presence of exogenous molybdenum. Loss of cofactor activity in vivo was not due to a defect in molybdenum uptake into the cells. In vitro nitrate reductase complementation between extracts of each of these four lines and a nia mutant showed that they possessed a functional nitrate reductase haemoflavoprotein subunit. Both constitutive molybdenum cofactor and NADH cytochrome c reductase activity were derepressed in the four cell lines. These results show that the four cell lines are indeed altered at a cnx locus, called cnxB, that the defect is probably in molybdenum processing and that there is a link between synthesis of functional molybdenum cofactor and nitrate reductase aporprotein.     

Nitrate reductase-deficient mutants in barley: enzyme stability and peptide mapping

Thermal stability and pH optima of NADH-nitrate reductase-associated cytochrome c reductase and FMNH₂-nitrate reductase from wild type, cv Steptoe or Winer, and mutants nar 1d, nar 1g, nar 1h, Xno 18 and Xno 19 were compared to determine if structural differences in the nitrate reductase protein could be detected. Also, the nitrate reductase-associated cytochrome c reductase from nar 1d was purified and compared with the wild type by peptide mapping. The pH optimum for FMNH₂-nitrate reductase from Steptoe and nar 1h, and for NADH-cytochrome c reductase from Steptoe, nar 1d, nar 1g and nar 2a was 7.5. Thermal stabilities of the nitrate reductase-associated activities (FMNH₂-nitrate reductase or NADH-cytochrome c reductase) from nar mutants were less than the Steptoe wild type, while Xno mutants were equal to the Winer wild type. Cleveland peptide maps of nar 1d NADH-cytochrome c reductase and Steptoe nitrate reductase were identicalwhen digested with endoprotease lys-C but were distinctly different in one peptide when digested with Staphylococcus aureus endoprotease V8. These results provide evidence that nar 1 gene codes for the nitrate reductase polypeptide.     

Isolation of molybdenum cofactor defective cell lines of Nicotiana tabacum

Summary Thirty-nine chlorate resistant cell lines were isolated after plating ethylmethane sulphonate treated allodihaploid cells of Nicotiana tabacum cv. Xanthi on agar medium containing 20 mM chlorate. Thirty-two of these cell lines grew as well on nitrate medium as on amino acid medium and three other cell lines grew well on amino acid medium but poorly on nitrate medium. Four other cell lines, 042, P12, P31 and P47 which could grow on amino acid medium, but not on nitrate medium, were examined further. They lacked in vitro nitrate reductase activity but were able to accumulate nitrate. All lines possessed nitrite reductase activity. Lines 042, P12, and P31 had a cytochrome c reductase species which was the same size as the wild type nitrate reductase associated cytochrome c reductase species, whilst the cytochrome c reductase species in line P47 was slightly smaller. All four lines lacked xanthine dehydrogenase activity and neither nitrate reductase nor xanthine dehydrogenase activity was restored by subculture of the four lines into either nitrate medium or glutamine medium supplemented with 1 mM sodium molybdate. These four lines are different from other molybdenum cofactor defective cell lines so far described in N. tabacum and possess similar properties to certain other cnx mutants described in Aspergillus nidulans.     

Induction of nitrate reductase and membrane cytochromes in wild type and chlorate-resistant Paracoccus denitrificans

An experimental system has been devised for induction of nitrate reductase in suspensions of wild type Paracoccus denitrificans incubated with limited aeration in the presence of azide, nitrate or nitrite. Azide promoted maximum synthesis of enzyme, accompanied by formation of excess b-type cytochrome; the level of enzyme attained with nitrate was less and c-type cytochrome predominated in the membrane. The nitrate reductase was solubilized with deoxycholate from membranes of azide-induced cells and was identified as a major polypeptide M _r =150,000 by sodium dodecyl sulphate-polyacrylamide gel electrophoresis. Mutants strains lacking nitrate reductase activity were isolated on the basis of resistance to chlorate and mutant M-1 was examined in detail. When incubated in the cell suspension system M-1 formed a membrane protein M _r =150,000 similar to that attributed to nitrate reductase in the wild type. Maximum formation of the protein by M-1 occurred without inducer and it was accompanied by synthesis of excess b-type cytochrome. The observations with wild type and M-1 indicate that nitrate reductase protein and b-type cytochrome are coregulated and that the active enzyme has a role in regulating its own synthesis.Non-standard Abbreviations SDS sodium dodecyl sulphate - PAGE polyacrylamide gel electrophoresis - DOC sodlum deoxycholate     

In vitro restoration of nitrate reductase: Investigation of Aspergillus nidulans and Neurospora crassa nitrate reductase mutants

Extracts of Aspergillus nidulans wild type (bi-1) and the nitrate reductase mutant niaD-17 were active in the in vitro restoration of NADPH-dependent nitrate reductase when mixed with extracts of Neurospora crassa, nit-1. Among the A. nidulans cnx nitrate reductase mutants tested, only the molybdenum repair mutant, cnxE-14 grown in the presence of 10⁻³ M Na₂MoO₄ was active in the restoration assay.Aspergillus extracts contained an inhibitor(s) which was measured by the decrease in NADPH-dependent nitrate reductase formed when extracts of Rhodospirillum rubrum and N. crassa, nit-1 were incubated at room temperature. The inhibition by extracts of A. nidulans, bi-1, cnxG-4 and cnxH-3 was a linear function of time and a logarithmic function of the protein concentration in the extract.The molybdenum content of N. crassa wild type and nit-1 mycelia were found to be similar, containing approx. 10 μg molybdenum/mg dry mycelium. The NADPH-dependent cytochrome c reductase associated with nitrate reductase was purified from both strains. The enzyme purified from wild-type N. crassa contained more than 1 mol of molybdenum per mol of enzyme, whereas the enzyme purified from nit-1 contained negligible amounts of molybdenum.     

Analysis of cytoplasmic membrane from wild type and mutant Paracoccus denitrificans containing excess nitrate reductase protein and cytochrome b

Cytoplasmic membranes were isolated from wild type and mutants strain M-1 of Paracoccus denitrificans grown with low aeration to promote synthesis of nitrate reductase protein and cytochrome b. The presence of 10-100-fold excess of nitrate reductase in the wild type or the corresponding enzymically inactive protein in the mutant did not significantly affect respiratory oxidase activities with NADH, succinate or TMPD-ascorbate as electron donor. A cytochrome b-nitrate reductase complex was resolved by isoelectric focussing of Triton X-100 solubilized membranes from the wild type grown with azide and from the mutant, whereas the enzyme complex from nitrate-grown wild type was not resolved from cytochrome c. Preparations from azideinduced wild type or from the mutant could be a suitable source of the cytochrome b associated with nitrate reductase for more detailed studies.Non standard abbreviations IEF isoelectric focussing - TMPD N, N, N, N-tetramethylphenylenediamine - SDS-PAGE Sodium dodecyl sulphate polyacrylamide gel electrophoresis     

In vitro reconstitution of NADH: nitrate reductase in nitrate reductase-deficient mutants of barley

Summary In vitro complementation of the nitrate reductase-deficient barley mutant nar2a extracts with molybdenum cofactor from commercial xanthine oxidase resulted in reactivation of NADH: nitrate reductase activity. Maximum reactivation was achieved with 7.5 g/ml xanthine oxidase (final concentration), 10 mM glutathione (final concentration) and incubation for 30 min at room temperature (ca. 25°C). This in vitro complementation assay was used to determine the presence of functional apoprotein and molybdenum cofactor in 12 nitrate reductase-deficient barley mutants. Extracts of all nar1 alleles contained functional molybdenum cofactor (complemented with nar2a) but they lacked functional apoprotein (did not complement with molybdenum cofactor from xanthine oxidase). The nar2a, nar3a and nar3b extracts were able to donate functional apoprotein, but were poor sources of functional molybdenum cofactor. These data are in agreement with our previous assignment of nar1 to the barley NADH: nitrate reductase structural locus and nar2 and nar3 to molybdenum cofactor functions. Wild type cv. Steptoe barley seedlings grown in the absence of nitrate and lacking nitrate reductase activity contained low levels of molybdenum cofactor. Nitrate induction resulted in a several-fold increase in the measurable molybdenum cofactor levels that was correlated with the increase in nitrate reductase activity.Scientific Paper No. 6839. College of Agriculture Research Center, Washington State University, Pullman. Project Nos. 0430 and 0233. This work was supported in part by National Science Foundation Grant PCM 81-19096 and USDA Competitive Research Grant 82-CRCR-1-1112     

Effects of molybdenum and tungsten on induction of nitrate reductase and formate dehydrogenase in wild type and mutant Paracoccus denitrificans

Molybdenum is required for induction of nitrate reductase and of NAD-linked formate dehydrogenase activities in suspensions of wild type Paracoccus denitrificans; tungsten prevents the development of these enzyme activities. The wild type forms a membrane protein M _r150,000 when incubated with tungsten and inducers of nitrate reductase and this is presumed to represent an inactive form of the enzyme. Suspensions of mutant M-1 did not develop nitrate reductase or formate dehydrogenase activities but the membrane protein M _r150,000 was formed under all conditions tested, including without inducers and without molybdenum. Analysis of membranes, solubilized with deoxycholate, by polyacrylamide gel electrophoresis under nondenaturing conditions showed that the mutant protein had similar electrophoretic mobility to the active nitrate reductase formed by the wilde type. Autoradiography of preparations from cells incubated with ⁵⁵Fe showed that the mutant and wild type proteins contained iron. However, in similar experiments with ⁹⁹Mo, incorporation of molybdenum into the mutant protein was not detectable.We conclude that mutant M-1 is defective in one or more steps required to process molybdenum for incorporation into molybdoenzymes. This failure affects the normal regulation of nitrate reductase protein with respect to the role of inducers.Non-Standard Abbreviations DOC deoxycholate - PAGE polyacrylamide gel electrophoresis - SDS sodium dodecyl sulfate     

Presence of the molybdenum-cofactor in nitrate reductase-deficient mutant cell lines of Nicotiana tabacum

Summary Nicotiana tabacum mutant cell cultures lacking nitrate reductase activity were assayed for the presence of the molybdenum-cofactor using its ability to restore NADPH-nitrate reductase activity in extracts of Neurospora crassa nit-1 mycelia. The molybdenum-cofactor of the tobacco wild-type line was shown to complement efficiently the N. crassa nit-1 mutant in vitro. The molybdenum-cofactor seems to exist in a bound form, as acid-treatment was required for release of cofactor activity. Molybdate (5–10 mM), ascorbic acid, and anaerobic conditions greatly increased the activity of the cofactor, demonstrating its high lability and sensitivity to oxygen. Similar results were obtained with two tobacco nia mutants, which are defective in the apoprotein of nitrate reductase. The four cnx mutants studied were shown to contain exclusively an inactive form of the molybdenum-cofactor. This inactive cofactor could be reactivated in vitro and in vivo by unphysiologically high concentrations of molybdate (1–10 mM), thereby converting the cnx cells into highly active cofactor sources in vitro, and restoring nitrate reductase and xanthine dehydrogenase in vivo to partial acitivity. Thus the defect of the cnx mutants resides in a lack of molybdenum as a catalytically active ligand metal for the cofactor, while the structural moiety of the cofactor seems not to be impaired by the mutation. The subunit assembly of the nitrate reductase was found to be independent of the molybdenum content of the cofactor.     

Nitrate reductase-deficient mutants in barley : immunoelectrophoretic characterization

Nitrate reductase-deficient barley (Hordeum vulgare L.) mutants were assayed for the presence of a functional molybdenum cofactor determined from the activity of the molybdoenzyme, xanthine dehydrogenase, and for nitrate reductase-associated activities. Rocket immunoelectrophoresis was used to detect nitrate reductase cross-reacting material in the mutants. The cross-reacting material levels of the mutants ranged from 8 to 136% of the wild type and were correlated with their nitrate reductase-associated activities, except for nar 1c, which lacked all associated nitrate reductase activities but had 38% of the wild-type cross-reacting material. The cross-reacting material of two nar 1 mutants, as well as nar 2a, Xno 18, Xno 19, and Xno 29, exhibited rocket immunoprecipitates that were similar to the wild-type enzyme indicating structural homology between the mutant and wild-type nitrate reductase proteins. The cross-reacting materials of the seven remaining nar 1 alleles formed rockets only in the presence of purified wild-type nitrate reductase, suggesting structural modifications of the mutant cross-reacting materials. All nar 1 alleles and Xno 29 had xanthine dehydrogenase activity indicating the presence of functional molybdenum cofactors. These results suggest that nar 1 is the structural gene for nitrate reductase. Mutants nar 2a, Xno 18, and Xno 19 lacked xanthine dehydrogenase activity and are considered to be molybdenum cofactor deficient mutants. Cross-reacting material was not detected in uninduced wild-type or mutant extracts, suggesting that nitrate reductase is synthesized de novo in response to nitrate.     

Nitrate reductase deficient cell lines from diploid cell cultures and lethal mutant M2 plants of Arabidopsis thaliana

Summary Cell suspensions of diploid Arabidopsis thaliana were screened for resistance to chlorate on a medium with ammonium nitrate as the nitrogen source, and after plating on filters to increase the plating efficiency. Thirty-nine lines were selected, four of which were still resistant after two years of subculturing on non-selective medium. Of the latter lines three were nitrate reductase deficient but exhibited some residual nitrate reductase activity; the fourth line showed a high level of enzyme activity. Screening M2-seeds for callus production on selective medium with amino acids as the nitrogen source and chlorate revealed resistant calli in 17 out of 483 M2-groups. Nine well-growing lines, all but one (G3) exhibiting no detectable in vivo nitrate reductase activity, were classified as defective in the cofactor. Two lines (G1 and G3) could be analysed genetically at the plant level. Chlorate resistance was monogenic and recessive. Sucrose gradient fractionation of callus extracts of G1 revealed that a complete enzyme molecule can be assembled. Nitrate reductase activity in G1 could partly be restored by excess molybdenum. It is suggested that G1 is disturbed in the catalytic properties of the cofactor. It appeared that G1 is neither allelic with another molybdenum repairable mutant (B73) nor with another cofactor mutant (B25). Wilting of intact G1 plants could be ascribed to non-closing stomata.     

Occurrence of xanthine dehydrogenase in Chlamydomonas reinhardii: A common cofactor shared by xanthine dehydrogenase and nitrate reductase

Wild-type Chlamydomonas reinhardii cells have xanthine dehydrogenase activity when grown with nitrate, nitrite, urea, or amino acid media. Mutant strains 102, 104, and 307 of Chlamydomonas, lacking both xanthine dehydrogenase and nitrate reductase activities, were incapable of restoring the NADPH-nitrate reductase activity of the mutant nit-1 of Neurospora crassa, whereas wild type cells and mutants 203 and 305 had xanthine dehydrogenase and were able to reconstitute the nitrate reductase activity of nit-1 of Neurospora. Therefore, it is concluded that in Chlamydomonas a common cofactor is shared by xanthine dehydrogenase and nitrate reductase. Xanthine dehydrogenase is repressed by ammonia and seems to be inessential for growth of Chlamydomonas.     

The influence of growth conditions on the synthesis of molybdenum cofactor in Proteus mirabilis

Cell-free extracts of Proteus mirabilis were able to reconstitute NADPH-dependent assimilatory nitrate reductase in crude extracts of the Neurospora crassa mutant strain nit-1, lacking molybdenum cofactor. Molybdenum cofactor was formed in the cytoplasm of the bacterium even in the presence of oxygen during growth though under these conditions no molybdo enzymes are formed. As a consequence no cofactor could be released by acid treatment from membranes of cells grown aerobically. The amount of cofactor released from membranes of cells grown anaerobically under various conditions was proportional to the amount of molybdo enzymes formed. During growth in the presence of tungstate a cofactor, which lacks molybdenum, was found in the cytoplasm. For detection of this so-called demolybdo cofactor the presence of molybdate during reconstitution was essential. Moreover, the cytoplasmic cofactor pool in cells grown in the presence of tungstate appeared to be two to three times higher than in cells grown under similar conditions without tungstate. After anaerobic growth in the presence of tungstate, the inactive demolybdo reductases were shown to contain partly no cofactor and partly a demolybdo cofactor. The P. mirabilis chlorate resistant mutant S 556 did not contain molybdenum cofactor. In two other chl-mutants the cofactor activity was the same as in the wild type.     

Complementation analysis of a nitrate reductase deficient Hyoscyamus muticus cell line by somatic hybridisation

Summary Complementation of a nitrate reductase deficient variant of Hyoscyamus muticus (MA-2) and nitrate reductase apoenzyme (nia-115) and cofactor mutants (cnx-68) of Nicotiana tabacum was studied by protoplast fusion. Selection of prototrophic intergeneric somatic hybrids was achieved in combination of MA-2 with the apoenzyme mutant nia-115 of N. tabacum. The H. muticus MA-2 line was therefore classified to be a cnx type variant possessing an altered molybdenum cofactor of the nitrate reductase enzyme complex but unaffected in the apoprotein of nitrate reductase. The nitrate reductase deficient and chlorate resistant characters of MA-2 were functionally coupled recessive traits. Nitrate reductase activity accompanied by chlorate sensitivity could be detected only under inductive conditions in the somatic hybrids. The inductive expression of nitrate reductase in the somatic hybrids arising from the combination of cells harbouring either the inductive or constitutive type nitrate reductase is discussed.Abbreviations DTT 1,4-Dithio-DL-threitol - Mo-co molybdenum containing cofactor - PEG polyethylene glycol     

Different forms of molybdenum cofactor inVicia faba seeds: The presence of molybdenum cofactor carrier protein and its purification

There were significant differences in the contents of molybdenum cofactor (Mo-co), both in a low-molecular-mass form (free Mo-co) and in a protein-bound form, in seeds of sevenVicia faba genotypes. Low-molecular-mass Mo-co species present in the extracts were detected by their ability to reactivate, through a dialysis membrane, aponitrate reductase from theNeurospora crassa nit-1 mutant. In extracts of all genotypes tested, the amount of Mo-co capable of directly reactivating nitrate reductase of theN. crassa nit-1 mutant was always much higher than that of low-molecular-mass Moco. These data cannot be explained by considering, as traditionally, that Mo-co detected directly, i.e. without any previous treatment for its release from Mo-coproteins, corresponds to free low-molecular mass Mo-co. A protein which bound Mo-co was purified to electrophoretic homogeneity. This protein consisted of a single 70-kDa polypeptide chain and carried a Mo-co that could be efficiently released when in contact with aponitrate reductase.Abbreviations CP carrier protein - Mo-co molybdenum cofactor - NR nitrate reductase - XO xanthine oxidase     

Nitrate reductase is not accumulated in chloroplast-ribosome-deficient mutants of higher plants

The activities of nitrite reductase (EC 1.7.7.1) are 60–70% of wild-type activity in pigment-deficient leaves of the chloroplast-ribosomedeficient mutants albostrians (Hordeum vulgare) and iojap (Zea mays). The activity and apoprotein of nitrate reductase (EC 1.6.6.1.) are lacking in the barley mutant. Only very low activities of nitrate reductase can be extracted from leaves of the maize mutant. The molybdenum cofactor of nitrate reductase and xanthine dehydrogenase (EC 1.2.3.2) is present in maize and barley mutant plants. However, it is not inducible by nitrate in pigment-deficient leaves of albostrians. From these results we conclude: (i) Nitrite reductase (a chloroplast enzyme) is synthesized in the cytoplasm and does not need the presence of nitrate reductase for the induction and maintenance if its activity. (ii) The loss or low activity of nitrate reductase is a consequence of the inability of the mutants to accumulate the apoprotein of this enzyme. (iii) The chloroplasts influence the accumulation (i.e. most probably the synthesis) of the nonchloroplast enzyme, nitrate reductase. The accumulation of nitrate reductase needs a chloroplast factor which is not provided by mutant plastids blocked at an early stage of their development.Abbreviations CRM cross-reacting material - Mo-co molybdenum cofactor - NiR nitrite reductase - NR nitrate reductase     

Involvement of the narJ and mob gene products in distinct steps in the biosynthesis of the molybdoenzyme nitrate reductase in Escherichia coli

The Escherichia coli mob locus is required for synthesis of active molybdenum cofactor, molybdopterin guanine dinucleotide. The mobB gene is not essential for molybdenum cofactor biosynthesis because a deletion of both mob genes can be fully complemented by just mobA. Inactive nitrate reductase, purified from a mob strain, can be activated in vitro by incubation with protein FA (the mobA gene product), GTP, MgCl₂, and a further protein fraction, factor X. Factor X activity is present in strains that lack MobB, indicating that it is not an essential component of factor X, but over-expression of MobB increases the level of factor X. MobB, therefore, can participate in nitrate reductase activation. The narJ protein is not a component of mature nitrate reductase but narJ mutants cannot express active nitrate reductase A. Extracts from narJ strains are unable to support the in vitro activation of purified mob nitrate reductase: they lack factor X activity. Although the mob gene products are necessary for the biosynthesis of all E. coli molybdoenzymes as a result of their requirement for molybdopterin guanine dinucleotide, NarJ action is specific for nitrate reductase A. The inactive nitrate reductase A derivative in a narJ strain can be activated in vitro following incubation with cell extracts containing the narJ protein. NarJ acts to activate nitrate reductase after molybdenum cofactor biosynthesis is complete.     

Non-coordinate expression of the neighbouring genes rplL and rpoB,C of Escherichia coli.

Summary Two types of nitrate reductase-deficient mutant cell lines (nia and cnx) of Nicotiana tabacum have been used for in vitro reconstitution of NADH-nitrate reductase. The cnx mutants simultaneously lack NADH-,FADH₂-, red benzyl viologen-nitrate reductase, and xanthine dehydrogenase activities, but retain the nitrate reductase-associated NADH-cytochrome c reductase activity. These mutants are interpreted to be defective in the molybdenum-containing cofactor necessary for nitrate reductase activity. In the nia lines xanthine dehydrogenase activity is unaffected, and the loss of NADH-nitrate reductase is accompanied by a loss of all partial activities of nitrate reductase, including NADH-cytochrome c reductase. When cnx cells (induced by nitrate) were homogenized together with nia cells (induced by nitrate or uninduced), NADH-nitrate reductase activity was detectable in the cell extract. No nitrate reductase was observed when the cnx mutants were homogenized together, or after cohomogenization of the nia mutants. Thus, the inactive nitrate reductase molecule formed in the cnx mutants has been complemented in vitro with the molybdenum-containing cofactor supplied by nia extracts, thus giving rise to NADH-nitrate reductase activity. This result gives additional support to the interpretation that the active nitrate reductase of Nicotiana tabacum is composed of at least the NADH-cytochrome c reductase moiety and a molybdenum-containing cofactor which is formed by the action of the cnx gene product(s).     

The correlation between the protein composition of cytoplasmic membranes and the formation of nitrate reductase A,chlorate reductase C and tetrathionate reductase in Proteus mirabilis wild type and some chlorate resistant mutants

Three genotypically different chlorate resistant mutants, chl I, chl II and chl III, appeared to lack completely nitrate reductase A, chlorate reductase C and tetrathionate reductase activity. Fumarate reductase is only partially affected in chl I and chl III and unaffected in chl II. Formate dehydrogenase is only partially diminished in chl II, hydrogenase is diminished in chl I and chl II and completely absent in chl III.Subunits of nitrate reductase A, chlorate reductase C and tetrathionate reductase have been identified in protein profiles of purified cytoplasmic membranes from the wild type and the three mutant strains, grown under various conditions. Only the presence and absence of the largest subunits of these enzymes appeared to be correlated with their repression and derepression in the wild type membranes. On the cytoplasmic membranes of the chl I and chl III mutants these subunits lack for the greater part. In the chl II mutant, however, these subunits are inserted in the membrane all together after anaerobic growth with or without nitrate.A model for the repression/derepression mechanism for the reductases has been proposed. It includes repression by cytochrome b components, whereas the redox-state of the nitrate reductase A molecule itself is also involved in its derepression under anaerobic conditions.