Tuning the Mycobacterium tuberculosis Alternative Sigma Factor SigF through the Multidomain Regulator Rv1364c and Osmosensory Kinase Protein Kinase D

The RsfSR two-component system regulates SigF function by monitoring the state of the respiratory electron transport chain in Mycobacterium smegmatis

In Mycobacterium smegmatis, the transcriptional activity of the alternative sigma factor SigF is posttranslationally regulated by the partner switching system consisting of SigF, the anti-SigF RsbW1, and three anti-SigF antagonists (RsfA, RsfB, and RsbW3). We previously demonstrated that expression of the SigF regulon is strongly induced in the Δaa3 mutant of M. smegmatis lacking the aa3 cytochrome c oxidase, the major terminal oxidase in the respiratory electron transport chain. Here, we identified and characterized the RsfSR two-component system involved in regulating the phosphorylation state of the major anti-SigF antagonist RsfB. RsfS (MSMEG_6130) is a histidine kinase with the cyclase/histidine kinase-associated sensing extracellular 3 domain at its N terminus, and RsfR (MSMEG_6131) is a receiver domain-containing protein phosphatase 2C-type phosphatase that can dephosphorylate phosphorylated RsfB. We demonstrated that phosphorylation of RsfR on Asp74 by RsfS reduces the phosphatase activity of RsfR toward phosphorylated RsfB and that the cellular abundance of the active unphosphorylated RsfB is increased in the Δaa3 mutant relative to the WT strain. We also demonstrated that the RsfSR two-component system is required for induction of the SigF regulon under respiration-inhibitory conditions such as inactivation of the cytochrome bcc1 complex and aa3 cytochrome c oxidase, as well as hypoxia, electron donor-limiting, high ionic strength, and low pH conditions. Collectively, our results reveal a key regulatory element involved in regulating the SigF signaling system by monitoring the state of the respiratory electron transport chain.

Giving a signal: how protein phosphorylation helps Bacillus navigate through different life stages

Protein phosphorylation is a universal mechanism regulating a wide range of cellular responses across all domains of life. The antagonistic activities of kinases and phosphatases can orchestrate the life cycle of an organism. The availability of bacterial genome sequences, particularly Bacillus species, followed by proteomics and functional studies have aided in the identification of putative protein kinases and protein phosphatases, and their downstream substrates. Several studies have established the role of phosphorylation in different physiological states of Bacillus species as they pass through various life stages such as sporulation, germination, and biofilm formation. The most common phosphorylation sites in Bacillus proteins are histidine, aspartate, tyrosine, serine, threonine, and arginine residues. Protein phosphorylation can alter protein activity, structural conformation, and protein-protein interactions, ultimately affecting the downstream pathways. In this review, we summarize the knowledge available in the field of Bacillus signaling, with a focus on the role of protein phosphorylation in its physiological processes.

Mycobacterial Regulatory Systems Involved in the Regulation of Gene Expression Under Respiration-Inhibitory Conditions

Mycobacterium tuberculosis is the causative agent of tuberculosis. M. tuberculosis can survive in a dormant state within the granuloma, avoiding the host-mounting immune attack. M. tuberculosis bacilli in this state show increased tolerance to antibiotics and stress conditions, and thus the transition of M. tuberculosis to the nonreplicating dormant state acts as an obstacle to tuberculosis treatment. M. tuberculosis in the granuloma encounters hostile environments such as hypoxia, nitric oxide, reactive oxygen species, low pH, and nutrient deprivation, etc., which are expected to inhibit respiration of M. tuberculosis. To adapt to and survive in respiration-inhibitory conditions, it is required for M. tuberculosis to reprogram its metabolism and physiology. In order to get clues to the mechanism underlying the entry of M. tuberculosis to the dormant state, it is important to understand the mycobacterial regulatory systems that are involved in the regulation of gene expression in response to respiration inhibition. In this review, we briefly summarize the information regarding the regulatory systems implicated in upregulation of gene expression in mycobacteria exposed to respiration-inhibitory conditions. The regulatory systems covered in this review encompass the DosSR (DevSR) two-component system, SigF partner switching system, MprBA-SigE-SigB signaling pathway, cAMP receptor protein, and stringent response.

Kinases on Double Duty: A Review of UniProtKB Annotated Bifunctionality within the Kinome

Phosphorylation facilitates the regulation of all fundamental biological processes, which has triggered extensive research of protein kinases and their roles in human health and disease. In addition to their phosphotransferase activity, certain kinases have evolved to adopt additional catalytic functions, while others have completely lost all catalytic activity. We searched the Universal Protein Resource Knowledgebase (UniProtKB) database for bifunctional protein kinases and focused on kinases that are critical for bacterial and human cellular homeostasis. These kinases engage in diverse functional roles, ranging from environmental sensing and metabolic regulation to immune-host defense and cell cycle control. Herein, we describe their dual catalytic activities and how they contribute to disease pathogenesis.

Stringent Response in Mycobacteria: From Biology to Therapeutic Potential

Mycobacterium tuberculosis is a human pathogen that can thrive inside the host immune cells for several years and cause tuberculosis. This is due to the propensity of M. tuberculosis to synthesize a sturdy cell wall, shift metabolism and growth, secrete virulence factors to manipulate host immunity, and exhibit stringent response. These attributes help M. tuberculosis to manage the host response, and successfully establish and maintain an infection even under nutrient-deprived stress conditions for years. In this review, we will discuss the importance of mycobacterial stringent response under different stress conditions. The stringent response is mediated through small signaling molecules called alarmones “(pp)pGpp”. The synthesis and degradation of these alarmones in mycobacteria are mediated by Rel protein, which is both (p)ppGpp synthetase and hydrolase. Rel is important for all central dogma processes—DNA replication, transcription, and translation—in addition to regulating virulence, drug resistance, and biofilm formation. Rel also plays an important role in the latent infection of M. tuberculosis. Here, we have discussed the literature on alarmones and Rel proteins in mycobacteria and highlight that (p)ppGpp-analogs and Rel inhibitors could be designed and used as antimycobacterial compounds against M. tuberculosis and non-tuberculous mycobacterial infections.

Insights into the evolutionary history of the virulent factor HBHA of Mycobacterium tuberculosis

In Mycobacterium tuberculosis, heparin-binding hemagglutinin (HBHAMT) has a relevant role in infection. It is also present in non-virulent mycobacteria and ancient actinobacteria, such as Rhodococcus opacus. To have a better understanding of the underlying mechanisms that shaped the evolutionary divergence of these proteins, we performed a comprehensive phylogenetic analysis of the regulatory sequences that drive the expression of hbha in saprophytic and pathogenic mycobacterial species. The alignment of the hbha loci showed the appearance of intergenic sequences containing regulatory elements upstream the hbha gene; this sequence arrangement is present only in slow-growing pathogenic mycobacteria. The heterologous expression of HBHAMT in oleaginous R. opacus PD630 results in protein binding to lipid droplets, as it happens with HBHA proteins from saprophytic mycobacteria. We hypothesize that mycobacterial hbha gene cluster underwent functional divergence during the evolutionary differentiation of slow-growing pathogenic mycobacteria. We propose here an evolutionary scenario to explain the structural and functional divergence of HBHA in fast and slow-growing mycobacteria.

The Partner Switching System of the SigF Sigma Factor in Mycobacterium smegmatis and Induction of the SigF Regulon Under Respiration-Inhibitory Conditions

The partner switching system (PSS) of the SigF regulatory pathway in Mycobacterium smegmatis has been previously demonstrated to include the anti-sigma factor RsbW (MSMEG_1803) and two anti-sigma factor antagonists RsfA and RsfB. In this study, we further characterized two additional RsbW homologs and revealed the distinct roles of three RsbW homologs [RsbW1 (MSMEG_1803), RsbW2 (MSMEG_6129), and RsbW3 (MSMEG_1787)] in the SigF PSS. RsbW1 and RsbW2 serve as the anti-sigma factor of SigF and the protein kinase phosphorylating RsfB, respectively, while RsbW3 functions as an anti-SigF antagonist through its protein interaction with RsbW1. Using relevant mutant strains, RsfB was demonstrated to be the major anti-SigF antagonist in M. smegmatis. The phosphorylation state of Ser-63 was shown to determine the functionality of RsfB as an anti-SigF antagonist. RsbW2 was demonstrated to be the only protein kinase that phosphorylates RsfB in M. smegmatis. Phosphorylation of Ser-63 inactivates RsfB to render it unable to interact with RsbW1. Our comparative RNA sequencing analysis of the wild-type strain of M. smegmatis and its isogenic Δaa₃ mutant strain lacking the aa₃ cytochrome c oxidase of the respiratory electron transport chain revealed that expression of the SigF regulon is strongly induced under respiration-inhibitory conditions in an RsfB-dependent way.