2. Discovery and Phenotype of Bacterial RMPs—RecFOR

In bacteria, RMPs are encoded by recF, recO, and recR genes which belong to the same DNA repair pathway (Table 1). Their discovery is the product of different studies. Horii and Clark were the first to use an Escherichia coli deletion mutant, encoding RecB, RecC, SbcB, and SbcC nucleases, thus allowed them to identify additional genes involved in homologous recombination (HR), including recF [1]. This genetic approach paved the way to identify other members of this pathway, eventually leading to the discovery of recO and recR [2,3]. Due to their phenotypic resemblances, recF, recO, and recR were grouped with other genes (recJ, recQ, recN, …) in the recF epistatic group. Genetic experiments revealing the similarity between SSB overexpression and recF deletion were critical to understand the role of the RecF pathway in SSB removal [4,5]. recFOR deletion mutants are generally characterized by functional defects related to RecA presynaptic nucleofilament (called RecA*) activities, such as: (1) conjugation defect, (2) UV, Gamma rays and drug sensitivity, (3) delayed SOS induction, (4) translesion synthesis (TLS) defects and (5) replication fork instability [1,2,3,6,7,8,9,10,11]. Altogether, these data support a mechanism in which the three bacterial RMPs (RecF, RecO, and RecR) work in the same homologous recombination repair pathway [12,13], mediated by ssDNA (also called gap repair). Interestingly, in bacteria such as Deinococcus radiodurans naturally lacking the recBC genes, the RecF pathway plays a significant role in double-strand break (DSB) repair through an extended synthesis-dependent strand annealing process [14].

Table 1. Overview of recombination mediators and partners in different organisms. This table lists RMPs, their interaction partners and functions in different organisms.

Phylogenetic analysis reveals considerable variability in recF, recO, and recR gene conservation despite their apparent epistasis [15]. In particular, recO or a recO ortholog gene is present in most bacterial species, whereas the recF gene is much less widely distributed [15,16]. This suggests either a redundancy of other DNA repair genes or a more complex mechanism in which RMPs have distinct functions but often cooperate with each other. Consistent with the second possibility, a rising number of studies revealed phenotypic differences between recO and recF in Bacillus subtilis and E. coli [17,18,19,20,21].

4. Discovery of the Eukaryotic RMP—RAD52/BRCA2

RAD52 was identified as a radiation-sensitive mutation in Saccharomyces cerevisiae [34]. Additional genetic studies revealed its implication in homologous recombination, working alongside other genes (Table 1), all belonging to the RAD52 epistatic group. Genes of this group encode several proteins organized into two subgroups: (1) members of the MRX complex, RAD50, MRE11, and XRS2 involved in the recognition, resection and protection of the DSB; (2) other proteins involved in HR, notably the recombinase RAD51 and RAD51 nucleofilament regulators (RAD54, RAD55, RAD57, RAD59 and TID1). Interestingly, a RAD52 deletion is 250-fold more sensitive to ionizing radiation compared to RAD51 [35]. This greater recombination defect is due to the loss of the second function of RAD52 in DNA repair, via its role in single-strand DNA annealing [36,37]. Although essential for HR in S. cerevisiae, the RAD52 deletion only presented a subtle HR deficient phenotype in vertebrates, suggesting the existence of another DNA repair mechanism in this phylum. Beyond this, recent work uncovered a new mitosis-specific functions of RAD52 in mammals, showing that RAD52 promotes a break-induced replication-like pathway, dubbed mitotic DNA synthesis, that allows completion of DNA replication during chronic stress [38,39,40]. This function appears to be crucial for maintaining of telomeres via recombination [41,42].

The breast cancer susceptibility gene BRCA2 is another crucial gene for HR in eukaryotes and was first identified in humans [43]. Analysis of truncated BRCA2 mutants in mouse uncovered its interaction with RAD51 [44]. The interaction between BRCA2 and the SSB-like protein RPA was established using Hela cells [45]. Together, these observations classify BRCA2 as a RMP facilitating homologous recombination at double-strand breaks during S and G2 phases of the cell cycle, when sister chromatid is available. In Ustilago maydis and mammalian cells, BRCA2 also acts as RMP during meiosis by facilitating the loading of DMC1 (RecA-like) onto ssDNA [46,47]. RAD52 was later found to be synthetically lethal to BRCA2 in humans despite the minor phenotype initially observed for the single-deletion [48]. RMP conservation from one organism to another is varied. Yeast only encodes RAD52, whereas Caenorhabditis elegans only encodes BRC2, mammalian cells encode both and show a different extent in HR phenotypes for the single BRCA2 and double knockdown BRCA2/RAD52 [48,49].