Eva I. Hyde

Structural and mechanistic studies of Escherichia coli nitroreductase with the antibiotic nitrofurazone. Reversed binding orientations in different redox states of the enzyme.

The antibiotics nitrofurazone and nitrofurantoin are used in the treatment of genitourinary infections and as topical antibacterial agents. Their action is dependent upon activation by bacterial nitroreductase flavoproteins, including the Escherichia coli nitroreductase (NTR). Here we show that the products of reduction of these antibiotics by NTR are the hydroxylamine derivatives. We show that the reduction of nitrosoaromatics is enzyme-catalyzed, with a specificity constant ∼10,000-fold greater than that of the starting nitro compounds. This suggests that the reduction of nitro groups proceeds through two successive, enzyme-mediated reactions and explains why the nitroso intermediates are not observed. The global reaction rate for nitrofurazone determined in this study is over 10-fold higher than that previously reported, suggesting that the enzyme is much more active toward nitroaromatics than previously estimated. Surprisingly, in the crystal structure of the oxidized NTR-nitrofurazone complex, nitrofurazone is oriented with its amide group, rather than the nitro group to be reduced, positioned over the reactive N5 of the FMN cofactor. Free acetate, which acts as a competitive inhibitor with respect to NADH, binds in a similar orientation. We infer that the orientation of bound nitrofurazone depends upon the redox state of the enzyme. We propose that the charge distribution on the FMN rings, which alters upon reduction, is an important determinant of substrate binding and reactivity in flavoproteins with broad substrate specificity.

NITROREDUCTASE: A PRODRUG‐ACTIVATING ENZYME FOR CANCER GENE THERAPY

1. The prodrug CB1954 (5‐(aziridin‐1‐yl)‐2,4‐dinitrobenzamide) is activated by Escherichia coli nitroreductase (NTR) to a potent DNA‐crosslinking agent.

A simple mechanism for co-dependence on two activators at an Escherichia coli promoter.

The Escherichia coli melAB promoter is co‐dependent upon two transcription activators, MelR and the cyclic AMP receptor protein, CRP. In this study we demonstrate positive co‐operativity between the binding of MelR and CRP at the melAB promoter, which provides a simple mechanism for its co‐dependence. MelR binds to four sites, centred at positions −42.5, −62.5, −100.5 and −120.5 relative to the melAB transcription start point. When MelR is pre‐bound, CRP is able to bind to a target located between MelR at positions −62.5 and −100.5. This increases the occupation of the two downstream sites for MelR, which is essential for transcription activation. We have identified residues within activating region 1 (AR1) of CRP that are important in transcription activation of the melAB promoter. At simple CRP‐dependent promoters, the surface of CRP containing these residues is involved in contacting the RNA polymerase α subunit. Our results show that, at the melAB promoter, the surface of CRP containing AR1 contacts MelR rather than RNA polymerase. Thus, MelR and CRP activate transcription by a novel mechanism in which they bind co‐operatively to adjacent sites and form a bacterial enhanceosome.

Transcription activation at the Escherichia coli melAB promoter: the role of MelR and the cyclic AMP receptor protein.

MelR is a melibiose‐triggered transcription activator that belongs to the AraC family of transcription factors. Using purified Escherichia coli RNA polymerase and a cloned DNA fragment carrying the entire melibiose operon intergenic region, we have demonstrated in vitro open complex formation and activation of transcription initiation at the melAB promoter. This activation is dependent on MelR and melibiose. These studies also show that the cyclic AMP receptor protein (CRP) interacts with the melAB promoter and increases MelR‐dependent transcription activation. DNAase I footprinting has been exploited to investigate the location of MelR‐and CRP‐binding sites at the melAB promoter. We showed previously that MelR binds to two identical 18 bp target sequences centred at position −100.5 (Site 1) and position −62.5 (Site 2). In this work, we show that MelR additionally binds to two other related 18 bp sequences: Site 1′, centred at position −120.5, located immediately upstream of Site 1, and Site R, at position −238.5, which overlaps the transcription start site of the divergent melR promoter. MelR can bind to Site 1′, Site 1, Site 2 and Site R, in both the absence and the presence of melibiose. However, in the presence of melibiose, MelR also binds to a fifth site (Site 2′, centred at position −42.5) located immediately downstream of Site 2, and overlapping the −35 region of the melAB promoter. Additionally, although CRP is unable to bind to the melAB promoter in the absence of MelR, in the presence of MelR, it binds to a site located between MelR binding Site 1 and Site 2. Thus, tandem‐bound MelR recruits CRP to the MelR. We propose that expression from the melAB promoter has an absolute requirement for MelR binding to Site 2′. Optimal expression of the melAB promoter requires Sites 1′, Site 1, Site 2 and Site 2′; CRP acts as a ‘bridge’ between MelR bound at Sites 1′ and 1 and at Sites 2 and 2′, increasing expression from the melAB promoter. In support of this model, we show that improvement of the base sequence of Site 2′ removes the requirement for Site 1′ and Site 1, and short circuits the effects of CRP.

E. coli NfsA: an alternative nitroreductase for prodrug activation gene therapy in combination with CB1954

Prodrug activation gene therapy is a developing approach to cancer treatment, whereby prodrug-activating enzymes are expressed in tumour cells. After administration of a non-toxic prodrug, its conversion to cytotoxic metabolites directly kills tumour cells expressing the activating enzyme, whereas the local spread of activated metabolites can kill nearby cells lacking the enzyme (bystander cell killing). One promising combination that has entered clinical trials uses the nitroreductase NfsB from Escherichia coli to activate the prodrug, CB1954, to a potent bifunctional alkylating agent. NfsA, the major E. coli nitroreductase, has greater activity with nitrofuran antibiotics, but it has not been compared in the past with NfsB for the activation of CB1954. We show superior in vitro kinetics of CB1954 activation by NfsA using the NADPH cofactor, and show that the expression of NfsA in bacterial or human cells results in a 3.5- to 8-fold greater sensitivity to CB1954, relative to NfsB. Although NfsB reduces either the 2-NO2 or 4-NO2 positions of CB1954 in an equimolar ratio, we show that NfsA preferentially reduces the 2-NO2 group, which leads to a greater bystander effect with cells expressing NfsA than with NfsB. NfsA is also more effective than NfsB for cell sensitisation to nitrofurans and to a selection of alternative, dinitrobenzamide mustard (DNBM) prodrugs.

Testing double mutants of the enzyme nitroreductase for enhanced cell sensitisation to prodrugs: effects of combining beneficial single mutations

Prodrug activation gene therapy for cancer involves expressing prodrug-activating enzymes in tumour cells, so they can be selectively killed by systemically administered prodrug. For example, Escherichia colinfsB nitroreductase (E.C. 1.6.99.7)(NTR), sensitises cells to the prodrug CB1954 (5-[aziridin-1-yl]-2,4-dinitrobenzamide), which it converts to a potent DNA-crosslinking agent. However, low catalytic efficiency with this non-natural substrate appears to limit the efficacy of this enzyme prodrug combination for eliminating the target cancer cells. To improve this, we aim to engineer NTR for improved prodrug activation. Previously, a number of single amino acid substitutions at six positions around the active site of the enzyme were found to increase activity, resulting in up to approximately 5-fold enhanced cell sensitisation to CB1954. In this study we have made pairwise combinations among some of the best mutants at each of these 6 sites. A total of 53 double mutants were initially screened in E. coli, then the 7 most promising were inserted into an adenovirus vector and compared in SKOV3 human ovarian carcinoma cells for sensitisation to CB1954 and two alternative prodrugs. The most effective mutants, T41L/N71S and T41L/F70A, were 14-17-fold more potent than WT NTR at sensitising the cancer cells to CB1954. The best mutant for activation of the dinitrobenzamide mustard prodrug SN23862 was T41L/F70A (4.8-fold improvement); and S40A/F124M showed 1.7-fold improvement over WT with the nitrobenzylphosphoramide mustard prodrug LH7. In two tumour xenograft models using SKOV3 or human prostate carcinoma PC3, T41L/N71S NTR demonstrated greater CB1954-dependent anti-tumour activity than WT NTR.

Steady-State and Stopped-Flow Kinetic Studies of Three Escherichia coli NfsB Mutants with Enhanced Activity for the Prodrug CB1954

The enzyme nitroreductase, NfsB, from Escherichia coli has entered clinical trials for cancer gene therapy with the prodrug CB1954 [5-(aziridin-1-yl)-2,4-dinitrobenzamide]. However, CB1954 is a poor substrate for the enzyme. Previously we made several NfsB mutants that show better activity with CB1954 in a cell-killing assay in E. coli. Here we compare the kinetic parameters of wild-type NfsB with CB1954 to those of the most active single, double, and triple mutants isolated to date. For wild-type NfsB the global kinetic parameters for both k(cat) and K(m) for CB1954 are about 20-fold higher than previously estimated; however, the measured specificity constant, k(cat)/K(m) is the same. All of the mutants are more active with CB1954 than the wild-type enzyme, the most active mutant showing about 100-fold improved specificity constant with CB1954 over the wild-type protein with little effect on k(cat). This enhancement in specificity constants for the mutants is not seen with the antibiotic nitrofurazone as substrate, leading to reversed nitroaromatic substrate selectivity for the double and triple mutants. However, similar enhancements in specificity constants are found with the quinone menadione. Stopped-flow kinetic studies suggest that the rate-determining step of the reaction is likely to be the release of products. The most active mutant is also selective for the 4-nitro group of CB1954, rather than the 2-nitro group, giving the more cytotoxic reduction product. The double and triple mutants should be much more effective enzymes for use with CB1954 in prodrug-activation gene therapy.

Repression of the Escherichia coli melR promoter by MelR: evidence that efficient repression requires the formation of a repression loop

The Escherichia coli MelR protein is a transcription activator that, in the presence of melibiose, activates expression of the melAB operon by binding to four sites located just upstream of the melAB promoter. MelR is encoded by the melR gene, which is expressed from a divergent transcript that starts 237 bp upstream of the melAB promoter transcript start point. In a recent study, we have identified a fifth DNA site for MelR that overlaps the melR promoter transcript start and −10 region. Here we show that MelR binding to this site can downregulate expression from the melR promoter; thus, MelR autoregulates its own expression. Optimal repression of the melR promoter is observed in the absence of melibiose and requires one of the four other DNA sites for MelR at the melAB promoter. The two MelR binding sites required for this optimal repression are separated by 177 bp. We suggest that, in the absence of melibiose, MelR forms a loop between these two sites. We argue that, in the presence of melibiose, this loop is broken as the melAB promoter is activated. However, in the presence of melibiose, the melR promoter can still be partially repressed by MelR binding to the site that overlaps the transcript start and −10 region. Parallels with the Escherichia coli araC–araBAD regulatory region are discussed.

Transcription activation at the Escherichia coli melAB promoter: interactions of MelR with its DNA target site and with domain 4 of the RNA polymerase σ subunit

Activation of transcription initiation at the Escherichia coli melAB promoter is dependent on MelR, a transcription factor belonging to the AraC family. MelR binds to 18 bp target sites using two helix–turn–helix (HTH) motifs that are both located in its C‐terminal domain. The melAB promoter contains four target sites for MelR. Previously, we showed that occupation of two of these sites, centred at positions −42.5 and −62.5 upstream of the melAB transcription start point, is sufficient for activation. We showed that MelR binds as a direct repeat to these sites, and we proposed a model to describe how the two HTH motifs are positioned. Here, we have used suppression genetics to confirm this model and to show that MelR residue 273, which is in HTH 2, interacts with basepair 13 of each target site. As our model for DNA‐bound MelR suggests that HTH 2 must be adjacent to the melAB promoter −35 element, we searched this part of MelR for amino acid side‐chains that might be able to interact with σ. We describe genetic evidence to show that MelR residue 261 is close to residues 596 and 599 of the RNA polymerase σ70 subunit, and that they can interact. Similarly, MelR residue 265 is shown to be able to interact with residue 596 of σ70. In the final part of the work, we describe experiments in which the MelR binding site at position −42.5 was improved. We show that this is detrimental to MelR‐dependent transcription activation because bound MelR is mispositioned so that it is unable to make ‘correct’ interactions with σ.

Binding of the Escherichia coli MelR protein to the melAB promoter: orientation of MelR subunits and investigation of MelR–DNA contacts

The  Escherichia  coli  MelR  protein  is  a  melibiose‐triggered transcription factor, belonging to the AraC family, that activates transcription initiation at the melAB promoter. Activation is dependent on the binding of MelR to four 18 bp sites, centred at position −42.5 (site 2′), position −62.5 (site 2), position −100.5 (site 1) and position −120.5 (site 1′) relative to the melAB transcription start point. Activation also depends on the binding of CRP to a single site located between MelR binding site 1 and site 2. All members of the AraC family contain two helix–turn–helix (HTH) motifs that contact two segments of the DNA major groove at target sites on the same DNA face. In this work, we have studied the binding of MelR to different sites at the melAB promoter, focusing on the orientation of binding of the two MelR HTH motifs, and the juxtaposition of the different bound MelR subunits with respect to each other. To do this, MelR was engineered to contain a single cysteine residue adjacent to either one or the other HTH motif. The MelR derivatives were purified, and the cysteine residues were tagged with p‐bromoacetamidobenzyl‐EDTA‐Fe, an inorganic DNA cleavage reagent. Patterns of DNA cleavage after MelR binding were then used to determine the positions of the two HTH motifs at target sites. In order to simplify our analysis, we exploited an engineered derivative of the melAB promoter in which MelR binding to site 2 and site 2′, in the absence of CRP, is sufficient for transcription activation. To assist in the interpretation of our results, we also used a shortened derivative of MelR, MelR173, that is able to bind to site 2 but not to site 2′. Our results show that MelR binds as a direct repeat to site 2 and site 2′ with the C‐terminal HTH located towards the promoter‐proximal end of each site. The orientation in which MelR binds to site 2′ appears to be determined by MelR–MelR interactions rather than by MelR–DNA interactions. In complementary experiments, we used genetic analysis to investigate the importance of different residues in the two HTH motifs of MelR. Epistasis experiments provided evidence that supports the proposed orientation of binding of MelR at its target site.