It is widely and reasonably believed that oxygen limitation, amino acid starvation and carbon source restriction are involved in establishing and maintaining Mycobacterium tuberculosis in a dormant state. Correspondingly, emergence from dormancy is related to a partial or complete amelioration of these conditions. The investigators' major hypothesis is that enzyme systems responding to and regulating the effect of these physiological conditions control the fate of the organism as it goes through the process of dormancy and recovery. They have identified three enzyme systems that comprise regulatory networks in MTb and are independent of a specific animal or culture model of dormancy: These are: 1) the ribonucleotide reductase system that reduces ribonucleotides to deoxyribonucleotides--the rate limiting step in DNA synthesis, 2) the cytochrome bd oxidase system involved in growth under microaerophilic conditions and, 3) the stringent response system, comprised of enzymes that synthesize and degrade guanosine tetra- and penta-phosphate, which regulate the synthesis of certain proteins and stable RNAs. The first two systems are oxygen responsive and the third is regulated by the availability of adequate amino acids and energy sources. The activity of the enzymes they are studying are regulated by physiological conditions relevant to the environment of dormant Mtb. In turn, the very nature of their enzymatic activity regulate the biochemical milieu in which the organism must survive.
Their specific aim i s to understand the molecular basis of the relationship between environmental conditions of dormancy and the protective enzymatic response. In addition to regulation at the protein level, they are interested in how the genes that encode these enzymes are regulated at the transcriptional level under various growth conditions and whether mutant strains of Mtb carrying a knock-out in the genes for these enzymes have the biochemically predicted phenotype in an animal model of Mtb dormancy. Understanding the biochemical and genetic regulation of these enzymes will provide important insights into the adaptive mycobacterial physiology that is necessary during critical phases of the life cycle of Mycobacterium tuberculosis.
