ABSTRACT
Although molybdenum is present at high concentration (10“2 mg L-1) in seawater [19], its distribution on land is uneven. Its concentration in soil water may be very low, particularly in acid soils with high iron contents. Even when molybdenum is present, biological activity may deplete the local concentration in an ecosystem; also, the concentration may be low when bacteria grow on a solid phase [92]. Bacteria have efficient systems for molybdate uptake and transport; this is composed of a periplasmic binding protein, an integral membrane protein, and an ‘ ‘energizer” protein. These three proteins are encoded by modA, modB, and modC genes, respec tively (see Sec. 1.2.2). The ModA, ModB, and ModC proteins from various organisms (e.g., E. coli, A. vinelandii, and Rhodobacter capsulatus) are very similar. Molybdate transport is strongly coupled to utilisation in E. coli and Klebsiella pneumoniae, but other nitrogen-fixing organisms appear to have a molybdenum storage protein. In all organisms studied so far, molybdate transport genes are regulated by a repressor protein, ModE. The ModE-[Mo04]2_ complex binds to the sequences TAYAT (Y = T or C) in the operator/promoter region in E. coli and prevents transcription of the modABCD operon [93].
