PROJECT SUMMARY
Mycobacterium tuberculosis (Mtb), the causative agent of tuberculosis (TB), remains the leading cause of death
amongst infectious causes. While we have made progress reducing TB mortality, disease burden has stagnated
due to our inability to reduce the incidence rate of TB. This can be attributed to our lack of transmission blocking
interventions and the highly infectious nature of Mtb’ s aerosol transmission. Aerosolization represents an
essential physiologic process for Mtb’ s transmission and infectious life cycle that can be targeted to reduce TB
transmission and incidence. Mtb has likely evolved an adaptive response to the dramatic microenvironment
changes it experiences as it transitions from the nutrient-rich lung to the desolate atmosphere. The specific
genes and metabolic pathways that Mtb employs to navigate these changes are poorly understood. In the aerosol
droplet environment, Mtb is deprived of CO2 and nitrogen sources which provide the carbon and nitrogen building
blocks on which Mtb relies for biosynthetic processes. My project seeks to understand metabolic mechanisms
underlying Mtb’ s response to CO2 and nitrogen deprivation. Preliminary data from our lab has identified arginine
biosynthesis as a key metabolic pathway mediating Mtb’ s response to CO2 limitation. I propose that arginine
biosynthesis mediates Mtb survival in the CO2 and nitrogen limited aerosol droplet environment, by maintaining
a steady flux of carbamoyl phosphate, thermodynamically activated and short-lived essential intermediate. In
this model, I propose that arginine biosynthesis is configured as a cycle mediated by ArcA or Rv2323c, a currently
uncharacterized protein which I hypothesize to act as an arginine dihydrolase. Through this configuration,
arginine biosynthesis couples NH4+ regeneration in ArcA or Rv2323c catalysis with the oxidative production of
CO2 in the tricarboxylic acid (TCA) cycle. I hypothesize that Mtb can regulate between Rv2323c and ArcA based
on nutrient availability. I predict that Mtb preferentially uses ArcA during CO2 and nitrogen limitation, since ArcA
can mediate an arginine cycle without consuming CO2 and NH4+ equivalents. I will test this hypothesis with three
Aims. In Aim 1, I will identify the biochemical role of Rv2323c in arginine metabolism through in vitro and in vivo
characterization of Rv2323c’s catalytic activity and physiologic function. In Aim 2, I will characterize the roles of
ArcA and Rv2323c in CO2 limited environments and measure their contributions to the endogenous pool of CO2
and impact on Mtb survival in air. In Aim 3, I will characterize the role of ArcA and Rv2323c in nitrogen limited
environments by determining Mtb’ s preference for either enzyme in these conditions and their impact on Mtb’ s
survival. These approaches will reveal novel insights into Mtb’ s adaptive response to aerosolization with the
potential of identifying therapeutic targets to block Mtb transmission.
