<p>The gene modD for a member of this family is found with molybdenum transport genes modABC in <taxon tax_id="1061">Rhodobacter capsulatus</taxon>. However, disruption of modD causes only a 4-fold (rather than 500-fold for modA, modB, modC) change in the external molybdenum concentration required to suppress an alternative nitrogenase. ModD proteins are highly similar to nicotinate-nucleotide pyrophosphorylase (also called quinolinate phosphoribosyltransferase). Their function is unknown. </p>