Species-Specific Interactions of Arr with RplK Mediate Stringent Response in Bacteria

Elucidating the role of c-di-AMP in Mycobacterium smegmatis: Phenotypic characterization and functional analysis

Cyclic-di-AMP (c-di-AMP) is an important secondary messenger molecule that plays a critical role in monitoring several important cellular processes, especially in several Gram-positive bacteria. In this study, we seek to unravel the physiological significance of the molecule c-di-AMP in Mycobacterium smegmatis under different conditions, using strains with altered c-di-AMP levels: c-di-AMP null mutant (ΔdisA) and a c-di-AMP over-expression mutant (Δpde). Our thorough analysis of the mutants revealed that the intracellular concentration of c-di-AMP could determine many basic phenotypes such as colony architecture, cell shape, cell size, membrane permeability etc. Additionally, it was shown to play a significant role in multiple stress adaptation pathways in the case of different DNA and membrane stresses. Our study also revealed how the biofilm phenotypes of M. smegmatis cells are altered with high intracellular c-di-AMP concentration. Next, we checked how c-di-AMP contributes to antibiotic resistance or susceptibility characteristics of M. smegmatis, which was followed by a detailed transcriptome profile analysis to reveal key genes and pathways such as translation, arginine biosynthesis, cell wall and plasma membrane are regulated by c-di-AMP in mycobacteria.

Kanglemycin A Can Overcome Rifamycin Resistance Caused by ADP-Ribosylation by Arr Protein

Rifamycins, such as rifampicin (Rif), are potent inhibitors of bacterial RNA polymerase (RNAP) and are widely used antibiotics. Rifamycin resistance is usually associated with mutations in RNAP that preclude rifamycin binding. However, some bacteria have a type of ADP-ribosyl transferases, Arr, which ADP-ribosylate rifamycin molecules, thus inactivating their antimicrobial activity. Here, we directly show that ADP-ribosylation abolishes inhibition of transcription by rifampicin, the most widely used rifamycin antibiotic. We also show that a natural rifamycin, kanglemycin A (KglA), which has a unique sugar moiety at the ansa chain close to the Arr modification site, does not bind to Arr from Mycobacterium smegmatis and thus is not susceptible to inactivation. We, found, however, that kanglemycin A can still be ADP-ribosylated by the Arr of an emerging pathogen, Mycobacterium abscessus. Interestingly, the only part of Arr that exhibits no homology between the species is the part that sterically clashes with the sugar moiety of kanglemycin A in M. smegmatis Arr. This suggests that M. abscessus has encountered KglA or rifamycin with a similar sugar modification in the course of evolution. The results show that KglA could be an effective antimicrobial against some of the Arr-encoding bacteria.

The Enzymes of the Rifamycin Antibiotic Resistome

Rifamycin antibiotics include the WHO essential medicines rifampin, rifabutin, and rifapentine. These are semisynthetic derivatives of the natural product rifamycins, originally isolated from the soil bacterium Amycolatopsis rifamycinica. These antibiotics are primarily used to treat mycobacterial infections, including tuberculosis. Rifamycins act by binding to the β-subunit of bacterial RNA polymerase, inhibiting transcription, which results in cell death. These antibiotics consist of a naphthalene core spanned by a polyketide ansa bridge. This structure presents a unique 3D configuration that engages RNA polymerase through a series of hydrogen bonds between hydroxyl groups linked to the naphthalene core and C21 and C23 of the ansa bridge. This binding occurs not in the enzyme active site where template-directed RNA synthesis occurs but instead in the RNA exit tunnel, thereby blocking productive formation of full-length RNA. In their clinical use to treat tuberculosis, resistance to rifamycin antibiotics arises principally from point mutations in RNA polymerase that decrease the antibiotic's affinity for the binding site in the RNA exit tunnel. In contrast, the rifamycin resistome of environmental mycobacteria and actinomycetes is much richer and diverse. In these organisms, rifamycin resistance includes many different enzymatic mechanisms that modify and alter the antibiotic directly, thereby inactivating it. These enzymes include ADP ribosyltransferases, glycosyltransferases, phosphotransferases, and monooxygenases.ADP ribosyltransferases catalyze group transfer of ADP ribose from the cofactor NAD⁺, which is more commonly deployed for metabolic redox reactions. ADP ribose is transferred to the hydroxyl linked to C23 of the antibiotic, thereby sterically blocking productive interaction with RNA polymerase. Like ADP ribosyltransferases, rifamycin glycosyl transferases also modify the hydroxyl of position C23 of rifamycins, transferring a glucose moiety from the donor molecule UDP-glucose. Unlike other antibiotic resistance kinases that transfer the γ-phosphate of ATP to inactivate antibiotics such as aminoglycosides or macrolides, rifamycin phosphotransferases are ATP-dependent dikinases. These enzymes transfer the β-phosphate of ATP to the C21 hydroxyl of the rifamycin ansa bridge. The result is modification of a critical RNA polymerase binding group that blocks productive complex formation. On the other hand, rifamycin monooxygenases are FAD-dependent enzymes that hydroxylate the naphthoquinone core. The result of this modification is untethering of the ansa chain from the naphthyl moiety, disrupting the essential 3D shape necessary for productive RNA polymerase binding and inhibition that leads to cell death.All of these enzymes have homologues in bacterial metabolism that either are their direct precursors or share common ancestors to the resistance enzyme. The diversity of these resistance mechanisms, often redundant in individual bacterial isolates, speaks to the importance of protecting RNA polymerase from these compounds and validates this enzyme as a critical antibiotic target.

ADP-ribosylation systems in bacteria and viruses

ADP-ribosylation is an ancient posttranslational modification present in all kingdoms of life. The system likely originated in bacteria where it functions in inter- and intra-species conflict, stress response and pathogenicity. It was repeatedly adopted via lateral transfer by eukaryotes, including humans, where it has a pivotal role in epigenetics, DNA-damage repair, apoptosis, and other crucial pathways including the immune response to pathogenic bacteria and viruses. In other words, the same ammunition used by pathogens is adapted by eukaryotes to fight back. While we know quite a lot about the eukaryotic system, expanding rather patchy knowledge on bacterial and viral ADP-ribosylation would give us not only a better understanding of the system as a whole but a fighting advantage in this constant arms race. By writing this review we hope to put into focus the available information and give a perspective on how this system works and can be exploited in the search for therapeutic targets in the future. The relevance of the subject is especially highlighted by the current situation of being amid the world pandemic caused by a virus harbouring and dependent on a representative of such a system.

Monomeric NADH-Oxidizing Methylenetetrahydrofolate Reductases from Mycobacterium smegmatis Lack Flavin Coenzyme

5,10-Methylenetetrahydrofolate reductase (MetF/MTHFR) is an essential enzyme in one-carbon metabolism for de novo biosynthesis of methionine. Our in vivo and in vitro analyses of MSMEG_6664/MSMEI_6484, annotated as putative MTHFR in Mycobacterium smegmatis, failed to reveal their function as MTHFRs. However, we identified two hypothetical proteins, MSMEG_6596 and MSMEG_6649, as noncanonical MTHFRs in the bacterium. MTHFRs are known to be oligomeric flavoproteins. Both MSMEG_6596 and MSMEG_6649 are monomeric proteins and lack flavin coenzymes. In vitro, the catalytic efficiency (k _cat/K_m ) of MSMEG_6596 (MTHFR1) for 5,10-CH₂-THF and NADH was ∼13.5- and 15.3-fold higher than that of MSMEG_6649 (MTHFR2). Thus, MSMEG_6596 is the major MTHFR. This interpretation was further supported by better rescue of the E. coli Δmthfr strain by MTHFR1 than by MTHFR2. As identified by liquid chromatography-tandem mass spectrometry, the product of MTHFR1- or MTHFR2-catalyzed reactions was 5-CH₃-THF. The M. smegmatis Δmsmeg_6596 strain was partially auxotrophic for methionine and grew only poorly without methionine or without being complemented with a functional copy of MTHFR1 or MTHFR2. Furthermore, the Δmsmeg_6596 strain was more sensitive to folate pathway inhibitors (sulfachloropyridazine, p-aminosalicylic acid, sulfamethoxazole, and trimethoprim). The studies reveal that MTHFR1 and MTHFR2 are two noncanonical MTHFR proteins that are monomeric and lack flavin coenzyme. Both MTHFR1 and MTHFR2 are involved in de novo methionine biosynthesis and required for antifolate resistance in mycobacteria.IMPORTANCE MTHFR/MetF is an essential enzyme in a one-carbon metabolic pathway for de novo biosynthesis of methionine. MTHFRs are known to be oligomeric flavoproteins. Our in vivo and in vitro analyses of Mycobacterium smegmatis MSMEG_6664/MSMEI_6484, annotated as putative MTHFR, failed to reveal their function as MTHFRs. However, we identified two of the hypothetical proteins, MSMEG_6596 and MSMEG_6649, as MTHFR1 and MTHFR2, respectively. Interestingly, both MTHFRs are monomeric and lack flavin coenzymes. M. smegmatis deleted for the major mthfr (mthfr1) was partially auxotroph for methionine and more sensitive to folate pathway inhibitors (sulfachloropyridazine, para-aminosalicylic acid, sulfamethoxazole, and trimethoprim). The studies reveal that MTHFR1 and MTHFR2 are novel MTHFRs involved in de novo methionine biosynthesis and required for antifolate resistance in mycobacteria.