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The maintenance of genome stability requires the coordinated actions of multiple proteins and protein complexes. One critical family of proteins is the recombination mediators. Their role is to facilitate the formation of recombinase nucleoprotein filaments on single-stranded DNA (ssDNA). Filament formation can take place on post-replicative ssDNA gaps as well as on 3′-tailed duplexes resulting from helicase–nuclease processing. In prokaryotes, the RecF, O, and R proteins are widely distributed and mediate RecA loading as either the RecFOR or RecOR complexes, depending on the species being studied. In this review, I compare and contrast the available biochemical and structural information to provide insight into the mechanism of action of this critical family of mediators.

The recombination mediator complex RecFOR, consisting of the RecF, RecO, and RecR proteins, is needed to initiate homologous recombination in bacteria by positioning the recombinase protein RecA on damaged DNA. Bacteria from the phylum Campylobacterota, such as the pathogen Campylobacter jejuni, lack the recF gene and trigger homologous recombination using only RecR and RecO. To elucidate the functional properties of C. jejuni RecR (cjRecR) in recombination initiation that differ from or are similar to those in RecF-expressing bacteria, we determined the crystal structure of cjRecR and performed structure-based binding analyses. cjRecR forms a rectangular ring-like tetrameric structure and coordinates a zinc ion using four cysteine residues, as observed for RecR proteins from RecF-expressing bacteria. However, the loop of RecR that has been shown to recognize RecO and RecF in RecF-expressing bacteria is substantially shorter in cjRecR as a canonical feature of Campylobacterota RecR proteins, indicating that cjRecR lost a part of the loop in evolution due to the lack of RecF and has a low RecO-binding affinity. Furthermore, cjRecR features a larger positive patch and exhibits substantially higher ssDNA-binding affinity than RecR from RecF-expressing bacteria. Our study provides a framework for a deeper understanding of the RecOR-mediated recombination pathway.

ABSTRACT The RecA protein is a recombinase functioning in recombinational DNA repair in bacteria. RecA is regulated at many levels. The expression of the recA gene is regulated within the SOS response. The activity of the RecA protein itself is autoregulated by its own C-terminus. RecA is also regulated by the action of other proteins. To date, these include the RecF, RecO, RecR, DinI, RecX, RdgC, PsiB, and UvrD proteins. The SSB protein also indirectly affects RecA function by competing for ssDNA binding sites. The RecO and RecR, and possibly the RecF proteins, all facilitate RecA loading onto SSB-coated ssDNA. The RecX protein blocks RecA filament extension, and may have other effects on RecA activity. The DinI protein stabilizes RecA filaments. The RdgC protein binds to dsDNA and blocks RecA access to dsDNA. The PsiB protein, encoded by F plasmids, is uncharacterized, but may inhibit RecA in some manner. The UvrD helicase removes RecA filaments from RecA. All of these proteins function in a network that determines where and how RecA functions. Additional regulatory proteins may remain to be discovered. The elaborate regulatory pattern is likely to be reprised for RecA homologues in archaeans and eukaryotes.

Recombination mediator proteins have come into focus as promising targets for cancer therapy, with synthetic lethal approaches now clinically validated by the efficacy of PARP inhibitors in treating BRCA2 cancers and RECQ inhibitors in treating cancers with microsatellite instabilities. Thus, understanding the cellular role of recombination mediators is critically important, both to improve current therapies and develop new ones that target these pathways. Our mechanistic understanding of BRCA2 and RECQ began in Escherichia coli. Here, we review the cellular roles of RecF and RecQ, often considered functional homologs of these proteins in bacteria. Although these proteins were originally isolated as genes that were required during replication in sexual cell cycles that produce recombinant products, we now know that their function is similarly required during replication in asexual or mitotic-like cell cycles, where recombination is detrimental and generally not observed. Cells mutated in these gene products are unable to protect and process replication forks blocked at DNA damage, resulting in high rates of cell lethality and recombination events that compromise genome integrity during replication.

The Escherichia coli RecF, RecO and RecR pro teins have previously been implicated in bacterial recombinational DNA repair at DNA gaps. The RecOR‐facilitated binding of RecA protein to single‐stranded DNA (ssDNA) that is bound by single‐stranded DNA‐binding protein (SSB) is much faster if the ssDNA is linear, suggesting that a DNA end (rather than a gap) facilitates binding. In addition, the RecOR complex facilitates RecA protein‐mediated D‐loop formation at the 5′ ends of linear ssDNAs. RecR protein remains associated with the RecA filament and its continued presence is required to prevent filament disassembly. RecF protein competes with RecO protein for RecR protein association and its addition destabilizes RecAOR filaments. An enhanced function of the RecO and RecR proteins can thus be seen in vitro at the 5′ ends of linear ssDNA that is not as evident in DNA gaps. This function is countered by the RecF/RecO competition for association with the RecR protein.

Hot compression deformation behaviors of medium carbon Cr-Ni-Mo Nb steel were investigated at deformation temperatures ranging from 1223 to 1423 K and strain rates of 0.1, 1 and 5 s^-1. Dynamic recovery （DRV） and dynamic recrystallization （DRX） were observed during the hot compression deformation. For all of the samples, DRX occurred at deformation temperatures above 1323 K at different strain rates, while below 1223 K, no DRX was observed. The activation energy of the tested steel was determined as 386.06 kJ/mol. The ratio of critical stress to peak stress and the ratio of critical strain to peak strain were 0. 835 and 0.37, respectively. Kinetic equations interpreting the DRX behavior of the tested steel were proposed, and the corresponding parameters including the volume fraction and the average grain size were determined. Moreover, the microstructures induced under different deformation conditions were analyzed.

RecR, together with RecF and RecO, facilitates RecA loading in the RecF pathway of homologous recombinational DNA repair in procaryotes . The human Rad52 protein is a functional counterpart of RecFOR. We present here the crystal structure of RecR from Deinococcus radiodurans (DR RecR). A monomer of DR RecR has a two‐domain structure: the N‐terminal domain with a helix–hairpin–helix (HhH) motif and the C‐terminal domain with a Cys 4 zinc‐finger motif, a Toprim domain and a Walker B motif. Four such monomers form a ring‐shaped tetramer of 222 symmetry with a central hole of 30−35 Å diameter. In the crystal, two tetramers are concatenated, implying that the RecR tetramer is capable of opening and closing. We also show that DR RecR binds to both dsDNA and ssDNA, and that its HhH motif is essential for DNA binding.

Helicobacter pylori infects the stomach of about half of the world's human population, frequently causing chronic inflammation at the origin of several gastric pathologies. One of the most remarkable characteristics of the species is its remarkable genomic plasticity in which homologous recombination (HR) plays a critical role. Here, we analyzed the role of the H. pylori homologue of the AddAB recombination protein. Bioinformatics analysis of the proteins unveils the similarities and differences of the H. pylori AddAB complex with respect to the RecBCD and AddAB complexes from Escherichia coli and Bacillus subtilis, respectively. Helicobacter pylori mutants lacking functional addB or/and addA show the same level of sensitivity to DNA-damaging agents such as UV or irradiation and of deficiency in intrachromosomal RecA-dependent HR. Epistasis analyses of both DNA repair and HR phenotypes, using double and triple recombination mutants, demonstrate that, in H. pylori, AddAB and RecOR complexes define two separate presynaptic pathways with little functional overlap. However, neither of these complexes participates in the RecA-dependent process of transformation of these naturally competent bacteria.

Alternative reproductive cycles make use of different strategies to generate different reproductive products. In Escherichia coli, recA and several other rec genes are required for the generation of recombinant genomes during Hfr conjugation. During normal asexual reproduction, many of these same genes are needed to generate clonal products from UV-irradiated cells. However, unlike conjugation, this latter process also requires the function of the nucleotide excision repair genes. Following UV irradiation, the recovery of DNA replication requires uvrA and uvrC, as well as recA, recF, and recR. The rec genes appear to be required to protect and maintain replication forks that are arrested at DNA lesions, based on the extensive degradation of the nascent DNA that occurs in their absence. The products of the recJ and recQ genes process the blocked replication forks before the resumption of replication and may affect the fidelity of the recovery process. We discuss a model in which several rec gene products process replication forks arrested by DNA damage to facilitate the repair of the blocking DNA lesions by nucleotide excision repair, thereby allowing processive replication to resume with no need for strand exchanges or recombination. The poor survival of cellular populations that depend on recombinational pathways (compared with that in their excision repair proficient counterparts) suggests that at least some of the rec genes may be designed to function together with nucleotide excision repair in a common and predominant pathway by which cells faithfully recover replication and survive following UV-induced DNA damage.

The recR gene product is necessary for homologous recombination and recombinational DNA repair in eubacteria. We report the isolation and sequencing of the recR gene from Streptomyces coelicolor. It encodes a protein of 198 amino acids, with a predicted molecular mass of 22 kDa. The deduced amino acid sequence shows significant similarity to that of RecR proteins from other bacteria, including Escherichia coli and Bacillus subtilis. Like these, Streptomyces RecR contains potential helix-hairpin-helix, zinc finger and ATP-binding motifs, as well as the Toprim domain which is present also in topoisomerases of Types IA and II, primases and nucleases of the OLD family. The recR genes of Escherichia coli and Bacillus subtilis are immediately preceded by a small ORF (orf12 and orf107, respectively). An equivalent ORF (orf1) is also found in Streptomyces. S. lividans recR mutants, obtained either by insertional inactivation of recR or by deletion of the gene together with the preceding ORF, displayed increased sensitivity to DNA-damaging agents (such as UV light and methylmethanesulfonate), when compared with the wild-type strain. Both mutants could be complemented by the wild-type orflrecR genes and also by the recR gene alone. Based on these results, orf1 appears to be dispensable for the repair function of Streptomyces RecR. In studies of heterologous complementation, the B. subtilis recR region (orf107recR) was found to complement the S. lividans deltaorflrecR mutant, but the equivalent region from E. coli (orf12recR) could not. However, in the absence of orf107, B. subtilis recR was unable to restore the wild-type phenotype to the Streptomyces deletion mutant.