Biochemical characterization of a singular mutant of nitrate reductase from Chlamydomonas reinhardii. New evidence for a heteropolymeric enzyme structure

A dual system formed by the ARC and NR molybdoenzymes mediates nitrite-dependent NO production in Chlamydomonas

Nitric oxide (NO) is a relevant signal molecule involved in many plant processes. However, the mechanisms and proteins responsible for its synthesis are scarcely known. In most photosynthetic organisms NO synthases have not been identified, and Nitrate Reductase (NR) has been proposed as the main enzymatic NO source, a process that in vitro is also catalysed by other molybdoenzymes. By studying transcriptional regulation, enzyme approaches, activity assays with in vitro purified proteins and in vivo and in vitro NO determinations, we have addressed the role of NR and Amidoxime Reducing Component (ARC) in the NO synthesis process. N\R and ARC were intimately related both at transcriptional and activity level. Thus, arc mutants showed high NIA1 (NR gene) expression and NR activity. Conversely, mutants without active NR displayed an increased ARC expression in nitrite medium. Our results with nia1 and arc mutants and with purified enzymes support that ARC catalyses the NO production from nitrite taking electrons from NR and not from Cytb5-1/Cytb5-Reductase, the component partners previously described for ARC (proposed as NOFNiR, Nitric Oxide-Forming Nitrite Reductase). This NR-ARC dual system would be able to produce NO in the presence of nitrate, condition under which NR is unable to do it.

Genetic analysis of nitrate reductase-deficient mutants in Chlamydomonas reinhardii

Six mutants (305, 301, 203, 307, 104 and 102) of Chlamydomonas reinhardii, all defective in nitrate reductase (NR) activity, have been genetically analyzed. All except 102 carry single Mendelian mutations.Mutant 305, defective in diaphorase activity and mutant 301, defective in terminal enzyme activity, did not give rise to wild-type recombinants when crossed to each other or with the nit-1 mutant isolated from strain 137c (which is actually a double mutant nit-1 nit-2). Nit-1 was shown to lack both diaphorase and terminal activities. Whether the mutated sites in 305 and 301 are located in a unique cistron (nit-1) or in two adjacent cistrons (nit-1a and nit-1b) coding for a diaphorase subunit and a terminal subunit of NR is discussed in the light of previous biochemical findings.The 203 mutation affecting a regulatory gene did not recombine with nit-2, the other mutated locus present in strain 137c.Mutants 307, 104 and 102, all lacking molybdenum cofactor for both NR and xanthine dehydrogenase, where shown to be affected in different loci. The genes mutated in 307 and 104 have been designated nit-3 and nit-4, respectively. The 102 strain is mutated in two non-linked loci, nit-5 and nit-6, with both mutations required to confer the mutant phenotype. One of these cryptic mutations is present in the "wild" strain 21gr.The results indicate that at least six or seven loci are involved in the production of an active NR enzyme: one (nit-1) or two (nit-1a and nit-1b) cistrons to produce the NR apoproteins responsible for the partial activities diaphorase and terminal, one locus (nit-2) for the regulation of NR synthesis, and four loci (nit-3, nit-4, nit-5 and nit-6) to produce the molybdenum cofactor. The loci nit-1a and nit-2 seem to correspond to the nit-A and nit-B loci described by Nichols and Syrett (J Gen Microbiol 108:71-77, 1978).

In vivo complementation analysis of nitrate reductase-deficient mutants in Chlamydomonas reinhardtii

In vivo complementation between different wild and mutant strains defective for nitrate assimilation has been performed by isolating diploid strains from the appropriate crosses. Twenty-two diploids homozygous or heterozygous with respect to nitrate reduction and able to grow on nitrate medium were obtained and their diploid character demonstrated from analyses of mating type, cell volume, nuclear size and progeny of crosses with haploid wild-type. All diploids were assayed for overall- and terminal-nitrate reductase (NR) activity and for the occurrence of the NR-diaphorase subunit. Data on NR activities in heterozygotes carrying mutation(s) in structural gene(s) (nit-1 or nit-1a, nit-1b) agree with the heteromultimeric nature of the enzyme complex previously described (Franco et al. (1984) EMBO J 3: 1403-1407), and indicate that subunits are exchangeable to form hybrid enzymes. In addition, in vitro complementation tests with mutant nit-1 of C. reinhardtii indicate that this mutant has defective NR-diaphorase subunits but intact terminal subunits. Super-repression caused by the mutant allele nit-2 is suppressed by the wild allele in heterozygotes, which suggests a positive control by the nit-2 product on structural gene(s) transcription. Mutant alleles of genes for the biosynthesis of molybdenum-containing cofactor, either nit-4 or nit-5 and nit-6, were recessive in diploids carrying them. The mutant allele of nit-3, from strain 307, was codominant in all heterozygotes suggesting that nit-3 codes for a protein whose activity is limiting for the molybdenum-cofactor biosynthetic pathway.