Hydrophobicity of Cholic Acid-Derived Amphiphiles Dictates the Antimicrobial Specificity.

The unique structural components of cell membranes of Gram-positive bacteria, Gram-negative bacteria, and mycobacteria provide an excellent therapeutic target for developing highly specific antimicrobials. Here, we report the synthesis of nine cholic acid (CA)-derived amphiphiles, where three hydroxyl groups of CA were tethered to dimethylamino pyridine and the C24-carboxyl group was conjugated with different alkyl chains. Structure-activity investigations revealed that amphiphile 1 harboring a methyl group has antimicrobial activity against mycobacterial species. On the other hand, amphiphile 7 containing an octyl chain was selective against Gram-positive and Gram-negative bacilli. Biochemical assays confirmed the selective membrane permeabilization abilities of amphiphiles 1 and 7. Importantly, we demonstrate the selective actions of amphiphiles in clearing biofilms, intracellular bacteria, and wound infections. Therefore, for the first time, we show that the unique structural features of CA-derived amphiphiles dictate selective activity against specific bacterial species.

Methylation of two-component response regulator MtrA in mycobacteria negatively modulates its DNA binding and transcriptional activation.

Post-translational modifications such as phosphorylation, nitrosylation, and pupylation modulate multiple cellular processes in Mycobacterium tuberculosis. While protein methylation at lysine and arginine residues is widespread in eukaryotes, to date only two methylated proteins in Mtb have been identified. Here, we report the identification of methylation at lysine and/or arginine residues in nine mycobacterial proteins. Among the proteins identified, we chose MtrA, an essential response regulator of a two-component signaling system, which gets methylated on multiple lysine and arginine residues to examine the functional consequences of methylation. While methylation of K207 confers a marginal decrease in the DNA-binding ability of MtrA, methylation of R122 or K204 significantly reduces the interaction with the DNA. Overexpression of S-adenosyl homocysteine hydrolase (SahH), an enzyme that modulates the levels of S-adenosyl methionine in mycobacteria decreases the extent of MtrA methylation. Most importantly, we show that decreased MtrA methylation results in transcriptional activation of mtrA and sahH promoters. Collectively, we identify novel methylated proteins, expand the list of modifications in mycobacteria by adding arginine methylation, and show that methylation regulates MtrA activity. We propose that protein methylation could be a more prevalent modification in mycobacterial proteins.

Functioning of Mycobacterial Heat Shock Repressors Requires the Master Virulence Regulator PhoP.

A hallmark feature of Mycobacterium tuberculosis pathogenesis lies in the ability of the pathogen to survive within macrophages under a stressful environment. Thus, coordinated regulation of stress proteins is critically important for an effective adaptive response of M. tuberculosis, the failure of which results in elevated immune recognition of the tubercle bacilli with reduced survival during chronic infections. Here, we show that virulence regulator PhoP impacts the global regulation of heat shock proteins, which protect M. tuberculosis against stress generated by macrophages during infection. Our results identify that in addition to classical DNA-protein interactions, newly discovered protein-protein interactions control complex mechanisms of expression of heat shock proteins, an essential pathogenic determinant of M. tuberculosis While the C-terminal domain of PhoP binds to its target promoters, the N-terminal domain of the regulator interacts with the C-terminal end of the heat shock repressors. Remarkably, our findings delineate a regulatory pathway which involves three major transcription factors, PhoP, HspR, and HrcA, that control in vivo recruitment of the regulators within the target genes and regulate stress-specific expression of heat shock proteins via protein-protein interactions. The results have implications on the mechanism of regulation of PhoP-dependent stress response in M. tuberculosisIMPORTANCE The regulation of heat shock proteins which protect M. tuberculosis against stress generated by macrophages during infection is poorly understood. In this study, we show that PhoP, a virulence regulator of the tubercle bacilli, controls heat shock-responsive genes, an essential pathogenic determinant of M. tuberculosis Our results unravel that in addition to classical DNA-protein interactions, complex mechanisms of regulation of heat shock-responsive genes occur through multiple protein-protein interactions. Together, these findings delineate a fundamental regulatory pathway where transcription factors PhoP, HspR, and HrcA interact with each other to control stress-specific expression of heat shock proteins.

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

Bacterial alternative sigma factors are mostly regulated by a partner-switching mechanism. Regulation of the virulence-associated alternative sigma factor SigF of Mycobacterium tuberculosis has been an area of intrigue, with SigF having more predicted regulators than other sigma factors in this organism. Rv1364c is one such predicted regulator, the mechanism of which is confounded by the presence of both anti-sigma factor and anti-sigma factor antagonist functions in a single polypeptide. Using protein binding and phosphorylation assays, we demonstrate that the anti-sigma factor domain of Rv1364c mediates autophosphorylation of its antagonist domain and binds efficiently to SigF. Furthermore, we identified a direct role for the osmosensor serine/threonine kinase PknD in regulating the SigF-Rv1364c interaction, adding to the current understanding about the intersection of these discrete signaling networks. Phosphorylation of SigF also showed functional implications in its DNA binding ability, which may help in activation of the regulon. In M. tuberculosis, osmotic stress-dependent induction of espA, a SigF target involved in maintaining cell wall integrity, is curtailed upon overexpression of Rv1364c, showing its role as an anti-SigF factor. Overexpression of Rv1364c led to induction of another target, pks6, involved in lipid metabolism. This induction was, however, curtailed in the presence of osmotic stress conditions, suggesting modulation of SigF target gene expression via Rv1364c. These data provide evidence that Rv1364c acts an independent SigF regulator that is sensitive to the osmosensory signal, mediating the cross talk of PknD with the SigF regulon.IMPORTANCEMycobacterium tuberculosis, capable of latently infecting the host and causing aggressive tissue damage during active tuberculosis, is endowed with a complex regulatory capacity built of several sigma factors, protein kinases, and phosphatases. These proteins regulate expression of genes that allow the bacteria to adapt to various host-derived stresses, like nutrient starvation, acidic pH, and hypoxia. The cross talk between these systems is not well understood. SigF is one such regulator of gene expression that helps M. tuberculosis to adapt to stresses and imparts virulence. This work provides evidence for its inhibition by the multidomain regulator Rv1364c and activation by the kinase PknD. The coexistence of negative and positive regulators of SigF in pathogenic bacteria reveals an underlying requirement for tight control of virulence factor expression.

Clathrin-Independent Killing of Intracellular Mycobacteria and Biofilm Disruptions Using Synthetic Antimicrobial Polymers.

Current membrane targeting antimicrobials fail to target mycobacteria due to their hydrophobic membrane structure, ability to form drug-resistant biofilms, and their natural intracellular habitat within the confines of macrophages. In this work, we describe engineering of synthetic antimicrobial polymers (SAMPs) derived from biocompatible polyamides that can target drug-sensitive and drug-resistant mycobacteria with high selectivity. Structure-activity relationship studies revealed that reduced hydrophobicity of cationic pendants induces enhanced and selective permeabilization of mycobacterial membranes. The least hydrophobic SAMP (TAC1) was found to be the most active with maximum specificity toward mycobacteria over E. coli, S. aureus, and mammalian cells. Membrane perturbation studies, scanning electron microscopy, and colony PCR confirmed the ability of TAC1 to induce membrane lysis and to bind to the genomic material of mycobacteria, thereby inducing mycobacterial cell death. TAC1 was most effective in perfusing and disrupting the mycobacterial biofilms and was also able to kill the intracellular mycobacteria effectively without inducing any toxicity to mammalian cells. Cellular uptake studies revealed clathrin independent uptake of TAC1, thereby allowing it to escape hydrolytic lysosomal degradation and effectively kill the intracellular bacteria. Therefore, this manuscript presents the design and selective antimycobacterial nature of polyamide polymers with charged hydrophobic pendants that have ability to disrupt the biofilms and kill intracellular mycobacteria.

The interaction of mycobacterial protein Rv2966c with host chromatin is mediated through non-CpG methylation and histone H3/H4 binding.

To effectively modulate the gene expression within an infected mammalian cell, the pathogen Mycobacterium tuberculosis would need to bring about epigenetic modifications at appropriate genomic loci. Working on this hypothesis, we show in this study that the mycobacterial protein Rv2966c is a 5-methylcytosine-specific DNA methyltransferase that is secreted out from the mycobacterium and gets localized to the nucleus in addition to the cytoplasm inside the host cell. Importantly, Rv2966c binds to specific DNA sequences, methylates cytosines predominantly in a non-CpG context and its methylation activity is positively influenced by phosphorylation. Interestingly, like the mammalian DNA methyltransferase, DNMT3L, Rv2966c can also interact with histone proteins. Ours is the first study that identifies a protein from a pathogenic bacteria with potential to influence host DNA methylation in a non-canonical manner providing the pathogen with a novel mechanism to alter the host epigenetic machinery. This contention is supported by repression of host genes upon M. tuberculosis infection correlated with Rv2966c binding and non-CpG methylation.

Regulation of homocysteine metabolism by Mycobacterium tuberculosis S-adenosylhomocysteine hydrolase.

Mycobacterium tuberculosis modulates expression of various metabolism-related genes to adapt in the adverse host environment. The gene coding for M. tuberculosis S-adenosylhomocysteine hydrolase (Mtb-SahH) is essential for optimal growth and the protein product is involved in intermediary metabolism. However, the relevance of SahH in mycobacterial physiology is unknown. In this study, we analyze the role of Mtb-SahH in regulating homocysteine concentration in surrogate host Mycobacterium smegmatis. Mtb-SahH catalyzes reversible hydrolysis of S-adenosylhomocysteine to homocysteine and adenosine and we demonstrate that the conserved His363 residue is critical for bi-directional catalysis. Mtb-SahH is regulated by serine/threonine phosphorylation of multiple residues by M. tuberculosis PknB. Major phosphorylation events occur at contiguous residues Thr219, Thr220 and Thr221, which make pivotal contacts with cofactor NAD⁺. Consequently, phosphorylation negatively modulates affinity of enzyme towards NAD⁺ as well as SAH-synthesis. Thr219, Thr220 and Thr221 are essential for enzyme activity, and therefore, responsible for SahH-mediated regulation of homocysteine.

Development of a new generation of vectors for gene expression, gene replacement, and protein-protein interaction studies in mycobacteria.

Escherichia coli-mycobacterium shuttle vectors are important tools for gene expression and gene replacement in mycobacteria. However, most of the currently available vectors are limited in their use because of the lack of extended multiple cloning sites (MCSs) and convenience of appending an epitope tag(s) to the cloned open reading frames (ORFs). Here we report a new series of vectors that allow for the constitutive and regulatable expression of proteins, appended with peptide tag sequences at their N and C termini, respectively. The applicability of these vectors is demonstrated by the constitutive and induced expression of the Mycobacterium tuberculosis pknK gene, coding for protein kinase K, a serine-threonine protein kinase. Furthermore, a suicide plasmid with expanded MCS for creating gene replacements, a plasmid for chromosomal integrations at the commonly used L5 attB site, and a hypoxia-responsive vector, for expression of a gene(s) under hypoxic conditions that mimic latency, have also been created. Additionally, we have created a vector for the coexpression of two proteins controlled by two independent promoters, with each protein being in fusion with a different tag. The shuttle vectors developed in the present study are excellent tools for the analysis of gene function in mycobacteria and are a valuable addition to the existing repertoire of vectors for mycobacterial research.

Signalling mechanisms in Mycobacteria.

The importance of inter- and intracellular signal transduction in all forms of life cannot be underestimated. A large number of genes dedicated to cellular signalling are found in almost all sequenced genomes, and Mycobacteria are no exception. What appears to be interesting in Mycobacteria is that well characterized signalling mechanisms used by bacteria, such as the histidine-aspartate phosphorelay seen in two-component systems, are found alongside signalling components that closely mimic those seen in higher eukaryotes. This review will describe the important contribution made by researchers in India towards the identification and characterization of proteins involved in two-component signalling, protein phosphorylation and cyclic nucleotide metabolism.

Phosphorylation of Mycobacterium tuberculosis Ser/Thr phosphatase by PknA and PknB.

BACKGROUND: The integrated functions of 11 Ser/Thr protein kinases (STPKs) and one phosphatase manipulate the phosphorylation levels of critical proteins in Mycobacterium tuberculosis. In this study, we show that the lone Ser/Thr phosphatase (PstP) is regulated through phosphorylation by STPKs. PRINCIPAL FINDINGS: PstP is phosphorylated by PknA and PknB and phosphorylation is influenced by the presence of Zn(2+)-ions and inorganic phosphate (Pi). PstP is differentially phosphorylated on the cytosolic domain with Thr(137), Thr(141), Thr(174) and Thr(290) being the target residues of PknB while Thr(137) and Thr(174) are phosphorylated by PknA. The Mn(2+)-ion binding residues Asp(38) and Asp(229) are critical for the optimal activity of PstP and substitution of these residues affects its phosphorylation status. Native PstP and its phosphatase deficient mutant PstP(c) (D38G) are phosphorylated by PknA and PknB in E. coli and addition of Zn(2+)/Pi in the culture conditions affect the phosphorylation level of PstP. Interestingly, the phosphorylated phosphatase is more active than its unphosphorylated equivalent. CONCLUSIONS AND SIGNIFICANCE: This study establishes the novel mechanisms for regulation of mycobacterial Ser/Thr phosphatase. The results indicate that STPKs and PstP may regulate the signaling through mutually dependent mechanisms. Consequently, PstP phosphorylation may play a critical role in regulating its own activity. Since, the equilibrium between phosphorylated and non-phosphorylated states of mycobacterial proteins is still unexplained, understanding the regulation of PstP may help in deciphering the signal transduction pathways mediated by STPKs and the reversibility of the phenomena.