Complementation analysis of nitrate reductase deficient mutants of Nicotiana tabacum by somatic hybridization.

Mutant cell lines lacking nitrate reductase activity were analyzed genetically. Protoplasts from one apoprotein defective (nia) and four cofactor defective (cnx) mutants were fused in all possible pairwise combinations with the aid of polyethylene glycol. Complementing hybrids were detected by their ability to grow with nitrate as sole nitrogen source and confirmed by measuring their nitrate reductase activity. Strong complementation was observed in all types of nia+cnx hybrids, whereas the cnx mutants failed to complement each other. From the results it can be concluded that the mutants studied are recessive and that the four cnx mutants are alleles of the same pair of duplicate loci (cnxA1, cnxA2).