Mycobacterium tuberculosis Rv0991c Is a Redox-Regulated Molecular Chaperone

Biosensor-integrated transposon mutagenesis reveals rv0158 as a coordinator of redox homeostasis in Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is evolutionarily equipped to resist exogenous reactive oxygen species (ROS) but shows vulnerability to an increase in endogenous ROS (eROS). Since eROS is an unavoidable consequence of aerobic metabolism, understanding how Mtb manages eROS levels is essential yet needs to be characterized. By combining the Mrx1-roGFP2 redox biosensor with transposon mutagenesis, we identified 368 genes (redoxosome) responsible for maintaining homeostatic levels of eROS in Mtb. Integrating redoxosome with a global network of transcriptional regulators revealed a hypothetical protein (Rv0158) as a critical node managing eROS in Mtb. Disruption of rv0158 (rv0158 KO) impaired growth, redox balance, respiration, and metabolism of Mtb on glucose but not on fatty acids. Importantly, rv0158 KO exhibited enhanced growth on propionate, and the Rv0158 protein directly binds to methylmalonyl-CoA, a key intermediate in propionate catabolism. Metabolite profiling, ChIP-Seq, and gene-expression analyses indicate that Rv0158 manages metabolic neutralization of propionate toxicity by regulating the methylcitrate cycle. Disruption of rv0158 enhanced the sensitivity of Mtb to oxidative stress, nitric oxide, and anti-TB drugs. Lastly, rv0158 KO showed poor survival in macrophages and persistence defect in mice. Our results suggest that Rv0158 is a metabolic integrator for carbon metabolism and redox balance in Mtb.

Redox-regulated chaperones in cell stress responses

Proteostasis and redox homeostasis are tightly interconnected and most protein quality control pathways are under direct redox regulation which allow cells to immediately respond to oxidative stress conditions. The activation of ATP-independent chaperones serves as a first line of defense to counteract oxidative unfolding and aggregation of proteins. Conserved cysteine residues evolved as redox-sensitive switches which upon reversible oxidation induce substantial conformational rearrangements and the formation of chaperone-active complexes. In addition to harnessing unfolding proteins, these chaperone holdases interact with ATP-dependent chaperone systems to facilitate client refolding and restoring proteostasis during stress recovery. This minireview gives an insight into highly orchestrated mechanisms regulating the stress-specific activation and inactivation of redox-regulated chaperones and their role in cell stress responses.

Transcriptional Response of Mycobacterium tuberculosis to Cigarette Smoke Condensate

Smoking is known to be an added risk factor for tuberculosis (TB), with nearly a quarter of the TB cases attributed to cigarette smokers in the 22 countries with the highest TB burden. Many studies have indicated a link between risk of active TB and cigarette smoke. Smoking is also known to significantly decrease TB cure and treatment completion rate and increase mortality rates. Cigarette smoke contains thousands of volatile compounds including carcinogens, toxins, reactive solids, and oxidants in both particulate and gaseous phase. Yet, to date, limited studies have analyzed the impact of cigarette smoke components on Mycobacterium tuberculosis (Mtb), the causative agent of TB. Here we report the impact of cigarette smoke condensate (CSC) on survival, mutation frequency, and gene expression of Mtb in vitro. We show that exposure of virulent Mtb to cigarette smoke increases the mutation frequency of the pathogen and strongly induces the expression of the regulon controlled by SigH—a global transcriptional regulator of oxidative stress. SigH has previously been shown to be required for Mtb to respond to oxidative stress, survival, and granuloma formation in vivo. A high-SigH expression phenotype is known to be associated with greater virulence of Mtb. In patients with pulmonary TB who smoke, these changes may therefore play an important, yet unexplored, role in the treatment efficacy by potentially enhancing the virulence of tubercle bacilli.

Mycobacterium tuberculosis VapC4 toxin engages small ORFs to initiate an integrated oxidative and copper stress response

The Mycobacterium tuberculosis (Mtb) VapBC4 toxin-antitoxin system is essential for the establishment of Mtb infection. Using a multitier, systems-level approach, we uncovered the sequential molecular events triggered by the VapC4 toxin that activate a circumscribed set of critical stress survival pathways which undoubtedly underlie Mtb virulence. VapC4 exclusively inactivated the sole transfer RNA^Cys (tRNA^Cys) through cleavage at a single site within the anticodon sequence. Depletion of the pool of tRNA^Cys led to ribosome stalling at Cys codons within actively translating messenger RNAs. Genome mapping of these Cys-stalled ribosomes unexpectedly uncovered several unannotated Cys-containing open reading frames (ORFs). Four of these are small ORFs (sORFs) encoding Cys-rich proteins of fewer than 50 amino acids that function as Cys-responsive attenuators that engage ribosome stalling at tracts of Cys codons to control translation of downstream genes. Thus, VapC4 mimics a state of Cys starvation, which then activates Cys attenuation at sORFs to globally redirect metabolism toward the synthesis of free Cys. The resulting newly enriched pool of Cys feeds into the synthesis of mycothiol, the glutathione counterpart in this pathogen that is responsible for maintaining cellular redox homeostasis during oxidative stress, as well as into a circumscribed subset of cellular pathways that enable cells to defend against oxidative and copper stresses characteristically endured by Mtb within macrophages. Our ability to pinpoint activation or down-regulation of pathways that collectively align with Mtb virulence-associated stress responses and the nonreplicating persistent state brings to light a direct and vital role for the VapC4 toxin in mediating these critical pathways.

Thiol-based switching mechanisms of stress-sensing chaperones

Thiol-based redox switches evolved as efficient post-translational regulatory mechanisms that enable individual proteins to rapidly respond to sudden environmental changes. While some protein functions need to be switched off to save resources and avoid potentially error-prone processes, protective functions become essential and need to be switched on. In this review, we focus on thiol-based activation mechanisms of stress-sensing chaperones. Upon stress exposure, these chaperones convert into high affinity binding platforms for unfolding proteins and protect cells against the accumulation of potentially toxic protein aggregates. Their chaperone activity is independent of ATP, a feature that becomes especially important under oxidative stress conditions, where cellular ATP levels drop and canonical ATP-dependent chaperones no longer operate. Vice versa, reductive inactivation and substrate release require the restoration of ATP levels, which ensures refolding of client proteins by ATP-dependent foldases. We will give an overview over the different strategies that cells evolved to rapidly increase the pool of ATP-independent chaperones upon oxidative stress and provide mechanistic insights into how stress conditions are used to convert abundant cellular proteins into ATP-independent holding chaperones.