MOLYBDATE TRANSPORT AND REGULATION IN BACTERIA

Molybdate is transported in bacteria by a high-affinity transport syst em composed of a periplasmic binding protein, an integral membrane pro tein, and an energizer protein. These three proteins are coded by modA , modB, and modC genes, respectively. The ModA, ModB, and ModC protein s from various organisms (Escherichia coli, Haemophilus influenzae, Az otobacter vinelandii, and Rhodobacter capsulatus) are very similar. Th e lowest K-m value reported for molybdate in the molybdate transport p rocess is approximately 50 nM. In a mod mutant, molybdate is transport ed by the sulfate transport system or by a nonspecific anion transport er. Molybdate transport is tightly coupled to utilization in E. coli a nd Klebsiella pneumoniae, while other dinitrogen-fixing organisms appe ar to have a molybdenum storage protein. In all organisms studied so f ar, molybdate transport genes are regulated by a repressor protein, Mo dE. The ModE-molybdate complex binds to the sequences TAYAT (Y = T or C) in the operator/promoter region in E. coli and prevents transcripti on of the modABCD operon. The ModE-molybdate complex binds to DNA as a homodimer in E. coli and possibly in other organisms as well. In R. c apsulatus, however, two ModE homologues (MopAB proteins) are required for repression.