Expression of alternative nitrogenases in Azotobacter vinelandii is repressed by molybdenum. Two strains with Tn5 insertion mutations showed alternative nitrogenase-dependent diazotrophic growth in the presence of Mo. The mutations were in a region which contained four open reading frames (ORFs 1-4). The genetic structure and predicted products of ORFs 2, 3 and 4 are typical of the membrane-associated elements of the ATP-binding cassette (ABC) superfamily of transport systems. The products of ORF3 and ORF4 are homologous with the products of the Escherichia coli genes chlD and the partially sequenced chlJ, respectively, both of which are implicated in molybdenum transport. ORF1, which is in the relative position of bacterial permease genes commonly specifying periplasmic binding proteins, encodes a 29 kDa protein with a novel primary structure. It lacks a potential signal sequence, and its C-terminal half consists of a tandem repeat of a segment which is homologous with the M(r) 7 kDa molybdenum-pterin binding protein Mop from Clostridium pasteurianum. This suggests that a substituted pterin may be involved in the initial capture or early metabolism of molybdenum.