New Mechanisms Of Para-Aminosalicylic Acid(PAS) Resistance In Mycobacterium Tuberculosis

The emergence and spread of drug resistant strains of Mycobacterium tuberculosis, combing the fact that there was no drug introduced to the clinical application, are limiting the choice of anti-tuberculosis antibiotics and comprise a serious threat to tuberculosis control. Therefore, it is necessary to reevaluate alternative older antibiotics used as antituberculous agents in past decades. Uderstanding of the action mechanism of M. tuberculosis drug and the drug resistacne mechanism of M. tuberculosis is important in tuberculosis control.The mechanistic basis for resistance of M. tuberculosis to para-aminosalicylic acid (PAS), an important agent in the treatment of multi-drug resistant tuberculosis (MDR-TB), has yet to be fully defined. As a substrate analog of the folate precursor para-aminobenzoic acid (PABA), PAS is ultimately bioactivated to hydroxydihydrofolate which inhibits dihydrofolate reductase (DHFR) and disrupts operation of fo late-dependent metabolic pathways. As a result, mutation of dihydrofolate synthase, an enzyme needed for bioactivation of PAS, causes PAS resistance in M. tuberculosis H37Rv.Here, we demonstrate that various missense mutations within the coding sequence of the dihydropteroate (H2Pte) binding pocket of dihydrofolate synthase (FolC) confer PAS resistance in laboratory isolates of M. tuberculosis and M. bovis. From a panel of85multi-drug resistant M. tuberculosis clinical isolates,5were found to harbor mutations in folC within the H2Pte binding pocket resulting in PAS resistance. While these alterations in the H2Pte binding pocket resulted in reduced dihydrofolate synthase activity, they also abolished bioactivation of hydroxydihydropteroate to hydroxydihydrofolate. Consistent with this model for abolished bioactivation, introduction of a wild type copy of folC fully restored PAS susceptibility in folC mutant strains. Confirmation of this novel PAS resistance mechanism will be beneficial for development of molecular based diagnostics for M. tuberculosis clinical isolates and for further defining the mode of action of this important tuberculosis drug.