Resistance to methotrexate due to AcrAB-dependent export from Escherichia coli

Many laboratory strains of Escherichia coli are resistant to methotrexate (MTX), a folate analogue that binds dihydrofolate reductase (DHFR). Mutations that inactivate either tolC or acrA confer MTX sensitivity. Further, overexpression of a fusion protein with DHFR activity reverses this sensitivity by titrating out intracellular MTX. These results suggest that MTX accumulates in cells where mutations in acrA or tolC have inactivated the TolC-dependent AcrAB multidrug resistance efflux pump.

Discrete promoter elements affect specific properties of RNA polymerase II transcription complexes

The frequency of transcription initiation at specific RNA polymerase II promoters is, in many cases, related to the ability of the promoter to recruit the transcription machinery to a specific site. However, there may also be functional differences in the properties of assembled transcription complexes that are promoter-specific or regulator-dependent and affect their activity. Transcription complexes formed on variants of the adenovirus major late (AdML) promoter were found to differ in several ways. Mutations in the initiator element increased the sarkosyl sensitivity of the rate of elongation and decreased the rate of early steps in initiation as revealed by a sarkosyl challenge assay that exploited the resistance of RNA synthesis to high concentrations of sarkosyl after formation of one or two phospho-diester bonds. Similar, but clearly distinct, effects were also observed after deletion of the binding site for upstream stimulatory factor from the AdML promoter. In contrast, deletion of binding sites for nuclear factor 1 and Oct-1, as well as mutations in the recognition sequence for initiation site binding protein, were without apparent effect on transcription complexes on templates containing the mouse mammary tumor virus promoter.

Chemically induced dimerization of dihydrofolate reductase by a homobifunctional dimer of methotrexate

BACKGROUND

Chemically induced dimerization (CID) can be used to manipulate cellular regulatory pathways from signal transduction to transcription, and to create model systems for study of the specific interactions between proteins and small-molecule chemical ligands. However, few CID systems are currently available. The properties of, and interactions between, Escherichia coli dihydrofolate reductase (DHFR) and the ligand methotrexate (MTX) meet many of the desired criteria for the development of a new CID system.

RESULTS

BisMTX, a homobifunctional version of MTX, was synthesized and tested for its ability to induce dimerization of DHFR. Gel-filtration analysis of purified DHFR confirmed that, in vitro, the protein was a monomer in the absence of dimerizer drug; in the presence of bisMTX, a complex of twice the monomeric molecular weight was observed. Furthermore, the off-rate was found to be 0.0002 s(-1), approximately 100 times slower than that reported for DHFR-MTX. Interestingly, the addition of excess bisMTX did not result in formation of the binary complex (1 protein:1 dimerizer) over the ternary complex (2 proteins:1 dimerizer), which suggests cooperative binding interactions (affinity modulation) between the two DHFR molecules in the bisMTX:DHFR(2) ternary complex.

CONCLUSIONS

The combination of DHFR and bisMTX provides a new CID system with properties that could be useful for applications in vivo. Formation of the bisMTX:DHFR(2) ternary complex in vitro is promoted over a wide range of dimerizer concentrations, consistent with the idea that formation of the ternary complex recruits energetically favorable interactions between the DHFR monomers in the complex.