The intrinsic ATPase activity of Mycobacterium tuberculosis UvrC is crucial for its damage-specific DNA incision function

The ATPase activity of yeast chromosome axis protein Hop1 affects the frequency of meiotic crossovers

Saccharomyces cerevisiae meiosis-specific Hop1, a structural constituent of the synaptonemal complex, also facilitates the formation of programmed DNA double-strand breaks and the pairing of homologous chromosomes. Here, we reveal a serendipitous discovery that Hop1 possesses robust DNA-independent ATPase activity, although it lacks recognizable sequence motifs required for ATP binding and hydrolysis. By leveraging molecular docking combined with molecular dynamics simulations and biochemical assays, we identified an ensemble of five amino acid residues in Hop1 that could potentially participate in ATP-binding and hydrolysis. Consistent with this premise, we found that Hop1 binds to ATP and that substitution of amino acid residues in the putative ATP-binding site significantly impaired its ATPase activity, suggesting that this activity is intrinsic to Hop1. Notably, K65A and N67Q substitutions in the Hop1 N-terminal HORMA domain synergistically abolished its ATPase activity, noticeably impaired its DNA-binding affinity and reduced its association with meiotic chromosomes, while enhancing the frequency of meiotic crossovers (COs). Overall, our study establishes Hop1 as a DNA-independent ATPase and reveals a potential biological function for its ATPase activity in the regulation of meiotic CO frequency.

Saccharomyces cerevisiae Rev7 promotes non-homologous end-joining by blocking Mre11 nuclease and Rad50's ATPase activities and homologous recombination

Recent studies have shown that, in human cancer cells, the tetrameric Shieldin complex (comprising REV7, SHLD1, SHLD2, and SHLD3) facilitates non-homologous end-joining (NHEJ) while blocking homologous recombination (HR). Surprisingly, several eukaryotic species lack SHLD1, SHLD2, and SHLD3 orthologs, suggesting that Rev7 may leverage an alternative mechanism to regulate the double-strand break (DSB) repair pathway choice. Exploring this hypothesis, we discovered that Saccharomyces cerevisiae Rev7 physically interacts with the Mre11-Rad50-Xrs2 (MRX) subunits, impedes G-quadruplex DNA synergized HU-induced toxicity, and facilitates NHEJ, while antagonizing HR. Notably, we reveal that a 42-amino acid C-terminal fragment of Rev7 binds to the subunits of MRX complex, protects rev7∆ cells from G-quadruplex DNA-HU-induced toxicity, and promotes NHEJ by blocking HR. By comparison, the N-terminal HORMA domain, a conserved protein-protein interaction module, was dispensable. We further show that the full-length Rev7 impedes Mre11 nuclease and Rad50's ATPase activities without affecting the latter's ATP-binding ability. Combined, these results provide unanticipated insights into the functional interaction between the MRX subunits and Rev7 and highlight a previously unrecognized mechanism by which Rev7 facilitates DSB repair via NHEJ, and attenuation of HR, by blocking Mre11 nuclease and Rad50's ATPase activities in S. cerevisiae.

Mycobacterium smegmatis putative Holliday junction resolvases RuvC and RuvX play complementary roles in the processing of branched DNA structures

In eubacteria, Holliday junction (HJ) resolvases (HJRs) are crucial for faithful segregation of newly replicated chromosomes, homologous recombination, and repair of stalled/collapsed DNA replication forks. However, compared with the Escherichia coli HJRs, little is known about their orthologs in mycobacterial species. A genome-wide analysis of Mycobacterium smegmatis identified two genes encoding putative HJRs, namely RuvC (MsRuvC) and RuvX (MsRuvX); but whether they play redundant, overlapping, or distinct roles remains unknown. Here, we reveal that MsRuvC exists as a homodimer while MsRuvX as a monomer in solution, and both showed high-binding affinity for branched DNAs compared with unbranched DNA species. Interestingly, the DNA cleavage specificities of MsRuvC and MsRuvX were found to be mutually exclusive: the former efficiently promotes HJ resolution, in a manner analogous to the Escherichia coli RuvC, but does not cleave other branched DNA species; whereas the latter is a versatile DNase capable of cleaving a variety of branched DNA structures, including 3' and 5' flap DNA, splayed-arm DNA and dsDNA with 3' and 5' overhangs but lacks the HJ resolution activity. Point mutations in the RNase H-like domains of MsRuvC and MsRuvX pinpointed critical residues required for their DNA cleavage activities and also demonstrated uncoupling between DNA-binding and DNA cleavage activities. Unexpectedly, we found robust evidence that MsRuvX possesses a double-strand/single-strand junction-specific endonuclease and ssDNA exonucleolytic activities. Combined, our findings highlight that the RuvC and RuvX DNases play distinct complementary, and not redundant, roles in the processing of branched DNA structures in M. smegmatis.

Macrophage activation highlight an important role for NER proteins in the survival, latency and multiplication of Mycobacterium tuberculosis

Nucleotide excision repair (NER) is one of the most extensively studied DNA repair processes in both prokaryotes and eukaryotes. The NER pathway is a highly conserved, ATP-dependent multi-step process involving several proteins/enzymes that function in a concerted manner to recognize and excise a wide spectrum of helix-distorting DNA lesions and bulky adducts by nuclease cleavage on either side of the damaged bases. As such, the NER pathway of Mycobacterium tuberculosis (Mtb) is essential for its survival within the hostile environment of macrophages and disease progression. This review focuses on present published knowledge about the crucial roles of Mtb NER proteins in the survival and multiplication of the pathogen within the macrophages and as potential targets for drug discovery.

Interrogating the substrate specificity landscape of UvrC reveals novel insights into its non-canonical function

Although it is relatively unexplored, accumulating data highlight the importance of tripartite crosstalk between nucleotide excision repair (NER), DNA replication, and recombination in the maintenance of genome stability; however, elucidating the underlying mechanisms remains challenging. While Escherichia coli uvrA and uvrB can fully complement polAΔ cells in DNA replication, uvrC attenuates this alternative DNA replication pathway, but the exact mechanism by which uvrC suppresses DNA replication is unknown. Furthermore, the identity of bona fide canonical and non-canonical substrates for UvrCs are undefined. Here, we reveal that Mycobacterium tuberculosis UvrC (MtUvrC) strongly binds to, and robustly cleaves, key intermediates of DNA replication/recombination as compared with the model NER substrates. Notably, inactivation of MtUvrC ATPase activity significantly attenuated its endonuclease activity, thus suggesting a causal link between these two functions. We built an in silico model of the interaction of MtUvrC with the Holliday junction (HJ), using a combination of homology modeling, molecular docking, and molecular dynamic simulations. The model predicted residues that were potentially involved in HJ binding. Six of these residues were mutated either singly or in pairs, and the resulting MtUvrC variants were purified and characterized. Among them, residues Glu595 and Arg597 in the helix-hairpin-helix motif were found to be crucial for the interaction between MtUvrC and HJ; consequently, mutations in these residues, or inhibition of ATP hydrolysis, strongly abrogated its DNA-binding and endonuclease activities. Viewed together, these findings expand the substrate specificity landscape of UvrCs and provide crucial mechanistic insights into the interplay between NER and DNA replication/recombination.

Mycobacteria excise DNA damage in 12- or 13-nucleotide-long oligomers by prokaryotic-type dual incisions and performs transcription-coupled repair

In nucleotide excision repair, bulky DNA lesions such as UV-induced cyclobutane pyrimidine dimers are removed from the genome by concerted dual incisions bracketing the lesion, followed by gap filling and ligation. So far, two dual-incision patterns have been discovered: the prokaryotic type, which removes the damage in 11-13-nucleotide-long oligomers, and the eukaryotic type, which removes the damage in 24-32-nucleotide-long oligomers. However, a recent study reported that the UvrC protein of Mycobacterium tuberculosis removes damage in a manner analogous to yeast and humans in a 25-mer oligonucleotide arising from incisions at 15 nt from the 3´ end and 9 nt from the 5´ end flanking the damage. To test this model, we used the in vivo excision assay and the excision repair sequencing genome-wide repair mapping method developed in our laboratory to determine the repair pattern and genome-wide repair map of Mycobacterium smegmatis We find that M. smegmatis, which possesses homologs of the Escherichia coli uvrA, uvrB, and uvrC genes, removes cyclobutane pyrimidine dimers from the genome in a manner identical to the prokaryotic pattern by incising 7 nt 5´ and 3 or 4 nt 3´ to the photoproduct, and performs transcription-coupled repair in a manner similar to E. coli.