Abstract
Most cyanobacteria are capable of nitrate assimilation and possess nitrate reductase (NR). In Synechococcus sp. strain PCC7942, three NR-related genetic loci, narA, narB, and narC, have been characterized. The narB locus contains the NR structural gene, narB, and the narA and narC loci contain genes involved in biosynthesis of the molybdenum cofactor, an essential prosthetic growth of NR. In addition to these we have identified another NR-related locus carrying a novel gene, narM, from a Synechococcus mutant that cannot grow in nitrate-containing medium. narM, encoding a small hydrophilic protein, has its homolog only in the cyanobacterial strains that are capable of nitrate assimilation. Insertional inactivation of the narM gene resulted in specific loss of the cellular NR activity, but did not effect the expression of narB and the molybdenum cofactor biosynthesis genes. These results suggested that a novel component is required for the functional expression of NR in cyanobacteria.
