MOLECULAR ANALYSIS OF THE MOLYBDATE UPTAKE OPERON, MODABCD, OF ESCHERICHIA-COLI AND MODR, A REGULATORY GENE

The nucleotide sequence of a 6.8-kb chromosomal subfragment of plasmid pHW100 complementing an Escherichia coli modC (chlD) mutant has been determined. This DNA region encodes the genes of a high-affinity uptak e system for molybdate arranged in an operon with the genes modABCD. S ince the modA product has a signal peptide at the N-terminus it probab ly is the periplasmic binding-protein for molybdate. The products of m odB (chlJ) and modC (chlD) have been described earlier as the inner me mbrane protein and the ATP-binding protein of the molybdate transport system, respectively. At present, there is no information on possible functions of the fourth gene of the operon, modD. Upstream of the mod operon, two other gene loci, termed modR and an open reading frame ORF 6 could be identified. ModR shares homology with a molybdenum-pterin b inding protein of Clostridium pasteurianum. ORF6 has extensive homolog y to ModC and other nucleotide-binding proteins of E. coli. Insertiona l inactivation of modR and ORF6 using a gentamicin resistance gene car tridge has no effect on molybdoenzyme activities, indicating that none of the two gene products is essential for molybdate uptake or molybde num cofactor synthesis. However, by using a plasmid carrying a modA-la cZ gene fusion we observed that inactivation of modR releases repressi on of the mod operon independent of the molybdate concentration in the medium. This indicates that modR is a component of the mod operon reg ulation or the repressor itself.