Hm. Walkenhorst et al., MOLECULAR ANALYSIS OF THE MOLYBDATE UPTAKE OPERON, MODABCD, OF ESCHERICHIA-COLI AND MODR, A REGULATORY GENE, Microbiological research, 150(4), 1995, pp. 347-361
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.