Mycobacterium tuberculosis (Mtb) is a leading cause of death, infecting and persisting in nearly one-third of the world’s population. It is believed that the immune response to Mtb infection causes much of the disease pathology. Although many people are infected with this bacterium, most do not display signs of disease. However, with the growing population of immunocompromised individuals, due to factors such as HIV infection, and the emergence of multi-drug resistant (MDR) strains of Mtb, the mortality due to tuberculosis is on the rise.

There is little question that Mtb requires many genes for dealing with the defenses of the mammalian immune system. One critical aspect of the immune system that is involved in fighting infection by Mtb is the production of nitric oxide (•NO or NO) by inducible nitric oxide synthase (iNOS/NOS2). NOS2 knock out mice are far more susceptible than isogenic wild type (wt) mice to death by Mtb. In humans, increasing evidence suggests that NOS2 expression is induced in macrophages from Mtb infected patients when compared to healthy, uninfected subjects. These observations strongly implicate a protective role for NO during a Mtb infection. Because RNI are produced by macrophages, the preferred residence of Mtb during an infection, Mtb has most likely developed mechanisms of resistance to RNI.

In order to identify the mechanisms by which Mtb resist killing by RNI, a genetic screen was performed. In this screen, 10,100 Mtb transposon insertion mutants were tested for their susceptibility to NO generated by acidified sodium nitrite (ASN, pH 5.5). From this library, we identified mutants with transposon insertions in two putative components (Mpa and Paf) of the Mtb proteasome. Proteasomes have been studied intensively in eukaryotic organisms, but little is understood about their function in bacteria. Although little is known about the function of proteasomes in prokaryotes, it is likely that they serve some similar functions, such as the degradation of damaged proteins. Furthermore, in the event the function of the proteasome is essential for the survival of Mtb when exposed to a particular stress, the proteasome may prove to be an ideal target for the development of new anti-tuberculosis drugs.

mpa and paf mutants are attenuated, even in immunodeficient (iNOS-/-) mice. This suggests that the mutants have other deficiencies that prevent them from being fully pathogenic. My lab’s research will determine how Mpa and Paf work with the proteasome to protect Mtb against RNI and other stresses in an animal host. We are in the process of identifying substrates of degradation as well as genes whose expression is dependent upon Mpa. Characterization of the Mtb proteasome will undoubtedly reveal some of the mechanisms by which the pathogen resists the immune system of the host.