Cleavage of the moaX-encoded fused molybdopterin synthase from Mycobacterium tuberculosis is necessary for activity

Amidation of glutamate residues in mycobacterial peptidoglycan is essential for cell wall cross-linking

Introduction

Mycobacteria assemble a complex cell wall with cross-linked peptidoglycan (PG) which plays an essential role in maintenance of cell wall integrity and tolerance to osmotic pressure. We previously demonstrated that various hydrolytic enzymes are required to remodel PG during essential processes such as cell elongation and septal hydrolysis. Here, we explore the chemistry associated with PG cross-linking, specifically the requirement for amidation of the D-glutamate residue found in PG precursors.

Methods

Synthetic fluorescent probes were used to assess PG remodelling dynamics in live bacteria. Fluorescence microscopy was used to assess protein localization in live bacteria and CRISPR-interference was used to construct targeted gene knockdown strains. Time-lapse microscopy was used to assess bacterial growth. Western blotting was used to assess protein phosphorylation.

Results and discussion

In Mycobacterium smegmatis, we confirmed the essentiality for D-glutamate amidation in PG biosynthesis by labelling cells with synthetic fluorescent PG probes carrying amidation modifications. We also used CRISPRi targeted knockdown of genes encoding the MurT-GatD complex, previously implicated in D-glutamate amidation, and demonstrated that these genes are essential for mycobacterial growth. We show that MurT-rseGFP co-localizes with mRFP-GatD at the cell poles and septum, which are the sites of cell wall synthesis in mycobacteria. Furthermore, time-lapse microscopic analysis of MurT-rseGFP localization, in fluorescent D-amino acid (FDAA)-labelled mycobacterial cells during growth, demonstrated co-localization with maturing PG, suggestive of a role for PG amidation during PG remodelling and repair. Depletion of MurT and GatD caused reduced PG cross-linking and increased sensitivity to lysozyme and β-lactam antibiotics. Cell growth inhibition was found to be the result of a shutdown of PG biosynthesis mediated by the serine/threonine protein kinase B (PknB) which senses uncross-linked PG. Collectively, these data demonstrate the essentiality of D-glutamate amidation in mycobacterial PG precursors and highlight the MurT-GatD complex as a novel drug target.

MoaE Is Involved in Response to Oxidative Stress in Deinococcus radiodurans

Molybdenum ions are covalently bound to molybdenum pterin (MPT) to produce molybdenum cofactor (Moco), a compound essential for the catalytic activity of molybdenum enzymes, which is involved in a variety of biological functions. MoaE is the large subunit of MPT synthase and plays a key role in Moco synthesis. Here, we investigated the function of MoaE in Deinococcus radiodurans (DrMoaE) in vitro and in vivo, demonstrating that the protein contributed to the extreme resistance of D. radiodurans. The crystal structure of DrMoaE was determined by 1.9 Å resolution. DrMoaE was shown to be a dimer and the dimerization disappeared after Arg110 had been mutated. The deletion of drmoaE resulted in sensitivity to DNA damage stress and a slower growth rate in D. radiodurans. The increase in drmoaE transcript levels the and accumulation of intracellular reactive oxygen species levels under oxidative stress suggested that it was involved in the antioxidant process in D. radiodurans. In addition, treatment with the base analog 6-hydroxyaminopurine decreased survival and increased intracellular mutation rates in drmoaE deletion mutant strains. Our results reveal that MoaE plays a role in response to external stress mainly through oxidative stress resistance mechanisms in D. radiodurans.

DRJAMM Is Involved in the Oxidative Resistance in Deinococcus radiodurans

Proteins containing JAB1/MPN/MOV34 metalloenzyme (JAMM/MPN+) domains that have Zn2+-dependent deubiquitinase (DUB) activity are ubiquitous across among all domains of life. Recently, a homolog in Deinococcus radiodurans, DRJAMM, was reported to possess the ability to cleave DRMoaD-MoaE. However, the detailed biochemical characteristics of DRJAMM in vitro and its biological mechanism in vivo remain unclear. Here, we show that DRJAMM has an efficient in vitro catalytic activity in the presence of Mn2+, Ca2+, Mg2+, and Ni2+ in addition to the well-reported Zn2+, and strong adaptability at a wide range of temperatures. Disruption of drJAMM led to elevated sensitivity in response to H2O2in vivo compared to the wild-type R1. In particular, the expression level of MoaE, a product of DRJAMM cleavage, was also increased under H2O2 stress, indicating that DRJAMM is needed in the antioxidant process. Moreover, DRJAMM was also demonstrated to be necessary for dimethyl sulfoxide respiratory system in D. radiodurans. These data suggest that DRJAMM plays key roles in the process of oxidative resistance in D. radiodurans with multiple-choice of metal ions and temperatures.

Analysis of Mycobacterium africanum in the last 17 years in Aragon identifies a specific location of IS6110 in Lineage 6

The purpose of this study was to increase our knowledge about Mycobacterium africanum and report the incidence and characteristics of tuberculosis (TB) due to their lineages in Aragon, Spain, over the period 2003–2019. The study includes all the cases in our region, where all the M. tuberculosis complex isolates are systematically characterised. We detected 31 cases of M. africanum among 2598 cases of TB in the period studied. TB caused by M. africanum is rare (1.19%) in our population, and it affects mainly men of economically productive age coming from West African countries. Among the isolates, Lineage (L) 6 was more frequent than L5. The genotyping of these strains identified five clusters and 13 strains with a unique pattern. The isolates’ characterisation identified a copy of IS6110 within the moaX gene, which turned out to be specific for L6. It will allow the differentiation of this lineage from the rest of MTBC with a simple PCR reaction. It remains to be established whether this polymorphism may limit M. africanum transmission. Furthermore, a mutation in the mutT2 promoter was found as specific for L6 strains, which could be related to the high variability found for L6 compared to L5.

Mycobacterium smegmatis does not display functional redundancy in nitrate reductase enzymes

Reduction of nitrate to nitrite in bacteria is an essential step in the nitrogen cycle, catalysed by a variety of nitrate reductase (NR) enzymes. The soil dweller, Mycobacterium smegmatis is able to assimilate nitrate and herein we set out to confirm the genetic basis for this by probing NR activity in mutants defective for putative nitrate reductase (NR) encoding genes. In addition to the annotated narB and narGHJI, bioinformatics identified three other putative NR-encoding genes: MSMEG_4206, MSMEG_2237 and MSMEG_6816. To assess the relative contribution of each, the corresponding gene loci were deleted using two-step allelic replacement, individually and in combination. The resulting strains were tested for their ability to assimilate nitrate and reduce nitrate under aerobic and anaerobic conditions, using nitrate assimilation and modified Griess assays. We demonstrated that narB, narGHJI, MSMEG_2237 and MSMEG_6816 were individually dispensable for nitrate assimilation and for nitrate reductase activity under aerobic and anaerobic conditions. Only deletion of MSMEG_4206 resulted in significant reduction in nitrate assimilation under aerobic conditions. These data confirm that in M. smegmatis, narB, narGHJI, MSMEG_2237 and MSMEG_6816 are not required for nitrate reduction as MSMEG_4206 serves as the sole assimilatory NR.

Structural analysis of molybdopterin synthases from two mycobacterial pathogens

The molybdenum cofactor, composed of molybdopterin and molybdenum, is a necessary compound for the catalytic activity of molybdenum enzymes. Molybdenum cofactor biosynthesis is a conserved multi-step process involving several enzymes. Molybdopterin synthase, a hetero-tetrameric enzyme composed of a pair of MoaE-MoaD subunits, catalyzes the generation of the cis-dithiolene group of molybdopterin in the second step of the process. The cis-dithiolene group can covalently bind molybdenum. Most mycobacterial species possess several genes encoding the full pathway of molybdenum cofactor biosynthesis. In M. smegmatis, the moaD2 and moaE2 genes encode the functional molybdopterin synthase. However, M. tuberculosis has genes encoding several molybdopterin synthase subunit homologs, including moaD1, moaD2, moaE1, moaE2, and moaX, which encodes a MoaD-MoaE fusion protein. Previous studies have shown that moaD2 and moaE2 encode functional molybdopterin synthase. Here, we report the crystal structures of two substrate-free molybdopterin synthases from two different mycobacterial pathogens, M. tuberculosis and M. smegmatis, at 2.1 Å and 2.6 Å resolutions, respectively. The overall structure of both molybdopterin synthases was hetero-tetrameric, consisting of a MoaE2 dimer flanked on either side by single MoaD2 subunits. The carboxyl-terminal domain of MoaD2 inserted into MoaE2, forming the active pocket. A comparison with previously reported molybdopterin synthase structures showed that substrate-binding and catalytic residues were conserved, despite low sequence similarity among these enzymes. The low sequence identity at the MoaE-MoaD heterodimer interface may provide the structural basis to explore mycobacterial inhibitors.

Cleavage of molybdopterin synthase MoaD-MoaE linear fusion by JAMM/MPN+ domain containing metalloprotease DR0402 from Deinococcus radiodurans

Molybdenum cofactor (Moco), molybdopterin (MPT) complexed with molybdenum, is an essential cofactor required for the catalytic center of diverse enzymes in all domains of life. Since Moco cannot be taken up as a nutrient unlike many other cofactors, Moco requires de novo biosynthesis. During the synthesis of MPT, the sulfur atom on the C-terminus of MoaD is transferred to cyclic pyranopterin monophosphate (cPMP) which is bound in the substrate pocket of MoaE. MoaD is a ubiquitin-like (Ubl) protein and has a C-terminal di-Gly motif which is a common feature of Ubl proteins. Despite the importance of free C terminal di-Gly motif of MoaD as a sulfur carrier, some bacteria encode a fused MPT synthase in which MoaD- and MoaE-like domains are located on a single peptide. Although it has recently been reported that the fused MPT synthase MoaX from Mycobacterium tuberculosis is posttranslationally cleaved into functional MoaD and MoaE in M. smegmatis, the protease responsible for the cleavage of MoaD-MoaE fusion protein has remained unknown to date. Here we report that the JAMM/MPN⁺ domain containing metalloprotease DR0402 (JAMM_DR) from Deinococcus radiodurans can cleave the MoaD-MoaE fusion protein DR2607, the sole MPT synthase in D. radiodurans, generating the MoaD having a C-terminal di-Gly motif. Furthermore, JAMM_DR can also cleave off the MoaD from MoaD-eGFP fusion protein suggesting that JAMM_DR recognizes the MoaD region rather than MoaE region in the cleaving process of MoaD-MoaE fusion protein.