Characterization of rifamycin‐resistant Mycobacterium tuberculosis RNA polymerases

Abstract

Tuberculosis (TB) is an infectious disease caused by Mycobacterium tuberculosis (MTB) where over 9 million people are infected globally and approximately 2 million lives are claimed annually. Rifampin (a semi‐synthetic rifamycin derivative) inhibits the MTB RNA polymerase (RNAP) and is one of the first‐line anti‐TB drugs. However rifamycin‐resistant (RifR) mutations are frequent and the individual substitutions of three β‐subunit residues of the RNAP (Asp435, His445, and Ser450) together account for 84% of MTB RifR strains found in clinical isolates. To better understand the interaction between rifamycins and wild‐type/RifR mutant MTB RNAPs, the IC50 values of known/novel rifamycins against RifR MTB RNAPs were determined. The inhibition of the different MTB RNAPs by rifamycins was investigated via dose‐response studies with an in vitro rolling circle transcription assay. Rifamycins bind tightly to the wild‐type MTB RNAP and inhibit the enzyme in the 10−9 M (nM) range. Whereas for the RifR mutants, a dramatic ~102–105 fold loss of affinity for rifamycins was observed where these different mutants were inhibited at much higher concentrations of rifamycins (10s–100s μM range). A direct binding assay is being developed to determine the equilibrium binding constant (Kd) values to further characterize the rifamycin·RNAP interactions.