Repression of the Escherichia coli modABCD (molybdate transport) operon by ModE

Abstract

The modABC gene products constitute the molybdate-specific transport system in Escherichia coli. Another operon coding for two proteins which diverges from the modABCD operon has been identified. The first gene of this operon codes for a 262-amino-acid protein, designated ModE (28 kDa The presence of molybdenum is required for the activity of several enzymes found in animals, plants, and bacteria, such as sulfite oxidase, xanthine dehydrogenase, nitrate reductase, formate dehydrogenase, and nitrogenase (46). In these organisms, the molybdenum is present in the molybdoenzymes in the form of a pterin-containing molybdenum cofactor (MoCo), with the exception of that found in nitrogenase, which has an ironmolybdenum cofactor (FeMoCo) (1,16,34).In Escherichia coli, the successful production of molybdoenzymes relies upon the efficient uptake of molybdate via the molybdate-specific transporter encoded by the modABCD operon (formerly known as the chlD locus) (18,26,35,42). The E. coli molybdate transport machinery, in which modA encodes a molybdate-specific periplasmic binding protein, modB encodes an integral membrane channel-forming protein, and modC encodes an ATP-binding energizer, closely resembles the established ATP-binding cassette (ABC) transporter motif (15, 26). Homologous molybdate transport systems have also been described for Azotobacter vinelandii, Rhodobacter capsulatus, and Haemophilus influenzae (9,23,45).Initial studies of the characterization of mod mutants revealed that strains harboring these mutations were incapable of producing nitrate reductase or formate dehydrogenase activity without molybdate supplementation (11,14,27,36,39,41,43). There are data to suggest that when mod mutants are grown in molybdate-supplemented media, molybdate enters the cells by means of the sulfate transport system as well as other nonspecific anion transporters (22,36). Some of the mod mutants were also analyzed for their molybdate uptake kinetics and were shown to transport molybdate at a much lower rate than mod ϩ strains did (5, 14). Further investigations of mod mutants have demonstrated that the modABCD operon is regulated by the intracellular concentration of molybdate. Specifically, high levels of molybdate reduced the level of transcription of the transport genes (27,35,36). This reduction in the level of transcription of the modABCD operon is most likely mediated by a molybdate-activated repressor protein.An analysis of the modABCD DNA sequence revealed a region in between the transcription and translation start sites of the modA gene which contains an 8-base inverted repeat (TAAC ⅐ GTTA) flanked by two CAT (CA) boxes (26,35,36). There are indications that either the CAT boxes or the inverted repeat or both are involved in the binding of the mod operon repressor (35). The inverted repeat found in this region is analogous to the inverted-repeat sequences implicated as the target binding sites for both the Met and Trp repressor (MetJ and TrpR) proteins (21,32,33).In our laboratory, a derivative of strain SE2069 ...

Functional characterization of the Bradyrhizobium japonicum modA and modB genes involved in molybdenum transport

“…In E. coli, expression of the mod operon also requires molybdate starvation (Rech et al, 1995;Rosentel et al, 1995). The repressor protein ModE binds molybdate to form a complex which, in turn, binds to the promoter region to inhibit transcription of mod genes (Grunden et al, 1996(Grunden et al, , 1999Anderson et al, 1997;McNicholas et al, 1998). A ModE-like protein has not been annotated in the genome sequence of B. japonicum (http://www.kazusa.jp/rhizobase/).…”

“…Expression of the modABCD operon of E. coli is tightly regulated and requires molybdate starvation. This regulation is achieved by a repressor protein, ModE, the product of the modE gene located in the modEF operon, which is transcribed divergently from modABCD (Grunden et al, 1996). ModE has a helix-turn-helix region in the N-terminal segment and is a member of the LysR family of transcriptional regulators.…”

“…ModE has a helix-turn-helix region in the N-terminal segment and is a member of the LysR family of transcriptional regulators. The ModE-molybdate complex is the active form of the protein and binds to target DNA as a dimer (Grunden et al, 1996(Grunden et al, , 1999Anderson et al, 1997). Although ModF has similarity to ModC, a mutation or deletion within the modF gene has no effect on molybdate uptake (Grunden et al, 1996).…”

Characterization of the ModE DNA‐binding sites in the control regions of modABCD and moaABCDE of Escherichia coli

“…In E. coli the modABCD operon is negatively regulated, in response to molybdate availability, by the product of the gene located immediately upstream, modE (Walkenhorst et al, 1995;Grunden et al, 1996;McNicholas et al, 1996). Parallel studies in R. capsulatus (Wang et al, 1993) and A. vinelandii (Mouncey et al, 1995) suggest a similar regulatory system and organization of the cognate regulators.…”

Expression, purification and DNA-binding activities of two putative ModE proteins of Herbaspirillum seropedicae (Burkholderiales, Oxalobacteraceae)

“…The ModE proteins regulate the transcription of modABC genes in response to molybdenum availability in E. coli and this regulation is based on the binding of the ModE-molybdate complex to a conserved binding sequence in the promoter region of the target genes (Grunden et al, 1996;Anderson et al, 1997;McNicholas et al, 1997;Grunden et al, 1999;Self et al, 2001). Furthermore, ModE also seems to regulate the expression of other genes involved with molybdenum (Anderson et al, 2000;McNicholas et al, 1998a;Tao et al, 2005).…”