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 gro wn on nitrate-containing media were isolated following nitrosoguanidin e mutagenesis, Six independent mutants were isolated, and the characte rization of one mutant, strain AMC260, which forms 6 to 8% heterocysts in the presence of nitrate, is presented. A 1.8-kb chromosomal fragme nt that complemented the AMC260 mutant was sequenced, and a 1.2-kb ope n reading frame, named moeA, was identified, The deduced amino acid se quence of the predicted Anabaena sp, strain PCC 7120 MoeA polypeptide shows 37% identity to MoeA from Escherichia coli, which is required fo r the synthesis of molybdopterin cofactor. Molybdopterin is required b y various molybdoenzymes, such as nitrate reductase, Interruption of t he moeA gene in Anabaena sp. strain PCC 7120 resulted in a strain, AMC 364, that showed a phenotype similar to that of AMC260, We show that A MC260 and AMC364 lack methyl viologen-supported nitrate reductase acti vity, 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.