Mannich reaction derivatives of novobiocin with modulated physiochemical properties and their antibacterial activities.

Synthetic derivatives of the natural product antibiotic novobiocin were synthesized in order to improve their physiochemical properties. A Mannich reaction was used to introduce new side chains at a solvent-exposed position of the molecule, and a diverse panel of functional groups was evaluated at this position. Novobiocin and the new derivatives were tested for their binding to gyrase B and their antibacterial activities against Staphylococcus aureus, Mycobacterium tuberculosis, Francisella tularensis and Escherichia coli. While the new derivatives still bound the gyrase B protein potently (0.07-1.8 µM, IC(50)), they had significantly less antibacterial activity. Two compounds were identified with increased antibacterial activity against M. tuberculosis, with a minimum inhibitory concentration of 2.5 µg/ml.

A high-throughput fluorescence polarization assay for inhibitors of gyrase B.

DNA gyrase, a type II topoisomerase that introduces negative supercoils into DNA, is a validated antibacterial drug target. The holoenzyme is composed of 2 subunits, gyrase A (GyrA) and gyrase B (GyrB), which form a functional A(2)B(2) heterotetramer required for bacterial viability. A novel fluorescence polarization (FP) assay has been developed and optimized to detect inhibitors that bind to the adenosine triphosphate (ATP) binding domain of GyrB. Guided by the crystal structure of the natural product novobiocin bound to GyrB, a novel novobiocin-Texas Red probe (Novo-TRX) was designed and synthesized for use in a high-throughput FP assay. The binding kinetics of the interaction of Novo-TRX with GyrB from Francisella tularensis has been characterized, as well as the effect of common buffer additives on the interaction. The assay was developed into a 21-µL, 384-well assay format and has been validated for use in high-throughput screening against a collection of Food and Drug Administration-approved compounds. The assay performed with an average Z' factor of 0.80 and was able to identify GyrB inhibitors from a screening library.

Studying the salt dependence of the binding of sigma70 and sigma32 to core RNA polymerase using luminescence resonance energy transfer.

The study of protein-protein interactions is becoming increasingly important for understanding the regulation of many cellular processes. The ability to quantify the strength with which two binding partners interact is desirable but the accurate determination of equilibrium binding constants is a difficult process. The use of Luminescence Resonance Energy Transfer (LRET) provides a homogeneous binding assay that can be used for the detection of protein-protein interactions. Previously, we developed an LRET assay to screen for small molecule inhibitors of the interaction of sigma70 with thebeta' coiled-coil fragment (amino acids 100-309). Here we describe an LRET binding assay used to monitor the interaction of E. coli sigma70 and sigma32 with core RNA polymerase along with the controls to verify the system. This approach generates fluorescently labeled proteins through the random labeling of lysine residues which enables the use of the LRET assay for proteins for which the creation of single cysteine mutants is not feasible. With the LRET binding assay, we are able to show that the interaction of sigma70 with core RNAP is much more sensitive to NaCl than to potassium glutamate (KGlu), whereas the sigma32 interaction with core RNAP is insensitive to both salts even at concentrations >500 mM. We also find that the interaction of sigma32 with core RNAP is stronger than sigma70 with core RNAP, under all conditions tested. This work establishes a consistent set of conditions for the comparison of the binding affinities of the E.coli sigma factors with core RNA polymerase. The examination of the importance of salt conditions in the binding of these proteins could have implications in both in vitro assay conditions and in vivo function.

Minimal promoter systems reveal the importance of conserved residues in the B-finger of human transcription factor IIB.

The "B-finger" of transcription factor IIB (TFIIB) is highly conserved and believed to play a role in the initiation process. We performed alanine substitutions across the B-finger of human TFIIB, made change-of-charge mutations in selected residues, and substituted the B-finger sequence from other organisms. Mutant proteins were examined in two minimal promoter systems (containing only RNA polymerase II, TATA-binding protein, and TFIIB) and in a complex system, using TFIIB-immunodepleted HeLa cell nuclear extract (NE). Mutations in conserved residues located on the sides of the B-finger had the greatest effect on activity in both minimal promoter systems, with mutations in residues Glu-51 and Arg-66 eliminating activity. The double change-of-charge mutant (E51R:R66E) did not show activity in either minimal promoter system. Mutations in the nonconserved residues at the tip of the B-finger did not significantly affect activity. However, all of the mutations in the B-finger showed at least 25% activity in the HeLa cell NE. Chimeric proteins, containing B-finger sequences from species with conserved residues on the side of the B-finger, showed wild-type activity in a minimal promoter system and in the HeLa cell NE. However, chimeric proteins whose sequence showed divergence on the sides of the B-finger had reduced activity. Transcription factor IIF (TFIIF) partially restored activity of the inactive mutants in the minimal promoter system, suggesting that TFIIF in HeLa cell NE helps to rescue the inactive mutations by interacting with either the B-finger or another component of the initiation complex that is influenced by the B-finger.

LRET-based HTS of a small-compound library for inhibitors of bacterial RNA polymerase.

Resistance mechanisms against whole classes of antibiotics are currently developing faster than research generates new structurally different biologically active agents. The demand for new antimicrobial drugs has not been matched by the speed of discovery. The interface between sigma and core of bacterial RNA polymerase offers an attractive target for drug discovery, and we have previously described the development of a very robust high-throughput assay for this target based on luminescence resonance energy transfer. Here we describe a semiautomated screen of a commercially available library (Chembridge, San Diego, CA) that led to the identification of four small molecules, two of which have activity in preventing in vitro transcription and growth of Escherichia coli.

A fast Western blot procedure improved for quantitative analysis by direct fluorescence labeling of primary antibodies.

The procedures for Western blots have been around for a long time and recent developments have increased the sensitivity for luminescent techniques so that the need for radioactive probes has been limited to only a few applications. Nevertheless, most protocols require more than 6 h and are often performed over more than a day. The majority of techniques require a secondary antibody conjugated to an enzyme that catalyzes a color reaction in order to amplify a detectable signal. However, both processes, the binding of a secondary antibody and the catalyzed reaction with the dye, are sources for errors and the latter is disadvantageous for a signal that is linear over a larger range of detected antigen. In order to improve the procedure most commonly used for quantitative analysis and convenience, we investigated the use of fluorescence labeling of primary monoclonal antibodies against Escherichia coli RNA polymerase subunits (beta', sigmaE and sigmaFecI) and their use in Western blots. We achieved a sensitivity (<1 ng detectable protein) comparable to most luminescent techniques. Additionally, we reduced the procedure time significantly to less than 1 h after SDS-PAGE and transfer to a membrane. Above all, we obtained a linear signal over the range of 30 ng to 1 microg of protein (dependent on protein size) making quantitative analysis of Western blots easier and more reliable.