Construction of an overexpression library for Mycobacterium tuberculosis

There is a pressing need to develop novel anti-tubercular drugs. High-throughput phenotypic screening yields chemical series that inhibit bacterial growth. Target identification for such series is challenging, but necessary for optimization of target engagement and the development of series into clinical drugs. We constructed a library of recombinant Mycobacterium tuberculosis strains each expressing a single protein from an inducible promoter as a tool for target identification. The library of 1733 clones was arrayed in 96-well plates for rapid screening and monitoring growth. The library contains the majority of the annotated essential genes as well as genes involved in cell wall and fatty acid biosynthesis, virulence factors, regulatory proteins, efflux, and respiration pathways. We evaluated the growth kinetics and plasmid stability over three passages for each clone in the library. We determined expression levels (mRNA and/or protein) in 396 selected clones. We screened the entire library and identified the Alr-expressing clone as the only recombinant strain, which grew in the presence of d-cycloserine (DCS). We confirmed that the Alr-expressing clone was resistant to DCS (7-fold shift in minimum inhibitory concentration). The library represents a new tool that can be used to screen for compound resistance and other phenotypes.

Mechanism of the AppABLUF Photocycle Probed by Site-Specific Incorporation of Fluorotyrosine Residues: Effect of the Y21 pKa on the Forward and Reverse Ground-State Reactions

The transcriptional antirepressor AppA is a blue light using flavin (BLUF) photoreceptor that releases the transcriptional repressor PpsR upon photoexcitation. Light activation of AppA involves changes in a hydrogen-bonding network that surrounds the flavin chromophore on the nanosecond time scale, while the dark state of AppA is then recovered in a light-independent reaction with a dramatically longer half-life of 15 min. Residue Y21, a component of the hydrogen-bonding network, is known to be essential for photoactivity. Here, we directly explore the effect of the Y21 pKa on dark state recovery by replacing Y21 with fluorotyrosine analogues that increase the acidity of Y21 by 3.5 pH units. Ultrafast transient infrared measurements confirm that the structure of AppA is unperturbed by fluorotyrosine substitution, and that there is a small (3-fold) change in the photokinetics of the forward reaction over the fluorotyrosine series. However, reduction of 3.5 pH units in the pKa of Y21 increases the rate of dark state recovery by 4000-fold with a Brønsted coefficient of ∼ 1, indicating that the Y21 proton is completely transferred in the transition state leading from light to dark adapted AppA. A large solvent isotope effect of ∼ 6-8 is also observed on the rate of dark state recovery. These data establish that the acidity of Y21 is a crucial factor for stabilizing the light activated form of the protein, and have been used to propose a model for dark state recovery that will ultimately prove useful for tuning the properties of BLUF photosensors for optogenetic applications.

Design, synthesis and biological evaluation of a highly-potent and cancer cell selective folate-taxoid conjugate

The folate receptor (FR) has been widely recognized as an excellent target for the tumor-selective delivery of cytotoxic agents, and four folate-drug conjugates have entered clinical evaluations for the treatment of solid tumors to date. However, most of these conjugates required structural modification of the cytotoxic warheads in order to achieve efficient drug release from the linkers. We designed and constructed a novel folate conjugate of a highly-potent next-generation taxoid, SB-T-1214, by exploiting bioorthogonal Cu-free 'click' chemistry. The synthesis was highly convergent and required no HPLC purification to obtain the final folate-taxoid conjugate 1. Conjugate 1 demonstrated highly FR-specific potency (IC₅₀ 2.1-3.5 nM) against a panel of cancer cell lines, with a >1000-fold decrease in cytotoxicity against normal human cells (IC₅₀>5000 nM). The remarkable potency and selectivity of conjugate 1 can be attributed to highly FR-specific receptor-mediated endocytosis as well as efficient release of the unmodified cytotoxic warhead using a mechanism-based self-immolative linker.

Isolation and characterization of compound-resistant isolates of Mycobacterium tuberculosis

This chapter describes the isolation and characterization of spontaneous resistant mutants of Mycobacterium tuberculosis. The overall objective of resistant mutant isolation is to determine the mode of action and/or cellular targets of new antimycobacterial agents. Whole-genome sequencing of resistant mutants can identify targets of antimycobacterial drugs and mechanisms of resistance, such as efflux, changes in drug permeability, or drug recognition. Mutants allow insight into in vivo biological processes and can help elucidate the number and identity of genes in a given pathway. Resistant mutant characterization can also lay the groundwork for structure/function studies, especially in conjunction with binding studies and X-ray crystallography.

Identification of the translational start site of codon-optimized mCherry in Mycobacterium tuberculosis

BACKGROUND

Fluorescent proteins are used widely as reporter genes in many organisms. We previously codon-optimized mCherry for Mycobacterium tuberculosis and generated expression constructs with high level expression in mycobacteria with multiple uses in vitro and in vivo. However, little is known about the expression of fluorescent proteins in mycobacteria and the translational start codon for mCherry has not been experimentally determined.

RESULTS

We determined the translational start site for functional (fluorescent) mCherry in mycobacteria. Several potential translational start codons were identified; introduction of downstream stop codons by mutagenesis was used to determine which start codon was utilized in the bacterial cells. Fluorescent protein was expressed from a construct which would allow translation of a protein of 226 amino acids or a protein of 235 amino acids. No fluorescence was seen when a construct which could give rise to a protein of 219 amino acids was used. Similar results were obtained in mycobacteria and in Escherichia coli. Western blotting confirmed that mCherry was expressed from the constructs encoding 235 or 226 amino acids, but not from the plasmid encoding 219 amino acids. N-terminal sequencing and mass determination confirmed that the mature protein was 226 amino acids and commenced with the amino acid sequence AIIKE.

CONCLUSION

We conclude that mCherry is expressed in M. tuberculosis as a smaller protein than expected lacking the GFP-derived N-terminal sequence designed to allow efficient fusions.