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).