Nitrate reductase activity and heterocyst suppression on nitrate in Anabaena sp. strain PCC 7120 require moeA

Mutants of Anabaena sp. strain PCC 7120 that form heterocysts when grown on nitrate-containing media were isolated following nitrosoguanidine mutagenesis. Six independent mutants were isolated, and the characterization of one mutant, strain AMC260, which forms 6 to 8% heterocysts in the presence of nitrate, is presented. A 1.8-kb chromosomal fragment that complemented the AMC260 mutant was sequenced, and a 1.2-kb open reading frame, named moeA, was identified. The deduced amino acid sequence of the predicted Anabaena sp. strain PCC 7120 MoeA polypeptide shows 37% identity to MoeA from Escherichia coli, which is required for the synthesis of molybdopterin cofactor. Molybdopterin is required by various molybdoenzymes, such as nitrate reductase. Interruption of the moeA gene in Anabaena sp. strain PCC 7120 resulted in a strain, AMC364, that showed a phenotype similar to that of AMC260. We show that AMC260 and AMC364 lack methyl viologen-supported nitrate reductase activity. We conclude that the inability of the moeA mutants to metabolize nitrate results in heterocyst formation on nitrate-containing media. Northern (RNA) analysis detected a 1.5-kb moeA transcript in wild-type cells grown in the presence or absence of a combined nitrogen source.