Molybdopterin guanine dinucleotide cofactor in Synechococcus sp. nitrate reductase: identification of mobA and isolation of a putative moeB gene.

The narC locus required for assimilatory nitrate reduction in the cyanobacterium Synechococcus sp. strain PCC 7942 was found to carry a mobA gene for molybdopterin guanine dinucleotide biosynthesis. Insertional inactivation of this gene blocked production of nitrate reductase in Synechococcus cells. We have previously described Synechococcus genes encoding homologues to molybdopterin biosynthesis proteins including MoaA, MoaC/MoaB, MoaD, MoaE, and MoeA, but not to MoeB. A cyanobacterial gene putatively encoding a protein composed of an amino-terminal domain of 260 amino acids homologous to Escherichia coli MoeB and of a carboxy-terminal extension of 130 amino acids was identified. Synechococcus mutants bearing only inactive versions of this putative moeB gene could not be isolated suggesting that it has function(s) additional to molybdopterin biosynthesis.