GENETIC-ANALYSIS OF THE MODABCD (MOLYBDATE TRANSPORT) OPERON OF ESCHERICHIA-COLI

DNA sequence analysis of the modABCD operon of Escherichia coli reveal ed the presence of four open reading frames. The first gene, modA, cod es for a 257-amino-acid periplasmic binding protein enunciated by the presence of a signal peptide-like sequence. The second gene (modB) enc odes a 229-amino-acid protein with a potential membrane location, whil e the 352-amino-acid ModC protein (modC product) contains a nucleotide -binding motif. On the basis of sequence similarities with proteins fr om other transport systems and molybdate transport proteins from other organisms, these three proteins are proposed to constitute the molybd ate transport system. The fourth open reading frame (modD) encodes a 2 31-amino-acid protein of unknown function. Plasmids containing differe nt mod genes were used to map several molybdate-suppressible chlorate- resistant mutants; interestingly, none of the 40 mutants tested had a mutation in the modD gene. About 35% of these chlorate-resistant mutan ts were not complemented by mod operon DNA. These mutants, designated mal, contained mutations at unknown chromosomal location(s) and produc ed formate hydrogenlyase activity only when cultured in molybdate-supp lemented glucose-minimal medium, not in L broth. This group of mol mut ants constitutes a new class of molybdate utilization mutants distinct from other known mutants in molybdate metabolism. These results show that molybdate, after transport into cells by the ModABC proteins, is metabolized (activated?) by the products of the mol gene(s).