# DisA Limits RecG Activities at Stalled or Reversed Replication Forks
## 1 Introduction  
Replicative DNA polymerase(s) may stall at obstacles (DNA lesions, RNA polymerases trafficking conflicts, etc. ), and then recombination functions are required to support replication restart [1, 2, 3]. Cells have evolved multiple mechanisms to prevent incorrect handling of perturbed replication forks that _per se_ do not compromise genome integrity. For instance, a replisome can simply skip a lesion in the lagging-strand template, leaving single-stranded (ss) DNA regions coated by the single-stranded binding protein (bacterial SSB/SsbA or eukaryotic RPA). An unrepaired lesion on the leading-strand template transiently uncouples the replicative DNA helicase from the stalled replicative DNA polymerase(s) and causes ssDNA regions coated by SSB/SsbA/RPA. This enables to circumvent the lesion by several DNA damage tolerance (DDT) sub-pathways [1, 2, 3, 4, 5]. When DNA replication was hindered in _Escherichia coli_ by mutations in some replication proteins, it was observed that the stalled replication fork reversed, with subsequent annealing of the nascent strands [6, 7, 5]. Replication fork reversal (also named fork regression), i.e., the active conversion of a stalled replication fork into a Holliday junction (HJ) structure, has emerged as a global and genetically controlled response to aid in the repair or bypass of DNA damage during replication stress [5, 8]. In _E. coli_, the enzymes that process stalled forks are RecA, RuvAB, RecG and RecQ [9, 10, 11, 12]. Here, the regressed arm of the reversed fork, which resembles a one-ended double-strand break (DSB), can be degraded by nucleases as the RecBCD complex (counterpart of _Bacillus subtilis_ AddAB), orthe HJ-like structure can be processed by the RuvABC complex, leading to fork breakage and repair by homologous recombination [13; 14]. Since during the first replication cycle of a reviving haploid _B. subtilis_ spore end resection functions have not been synthesized yet [15; 16] and there is only one copy of the replicating genome, a reversed fork (mimicking a one-ended DSB) may represent a pathological structure that can be lethal. Therefore, such fork remodeling should be regulated. A powerful tool to characterize the functions involved in replication fork remodeling and how they are controlled is to damage the DNA of inert non-replicating haploid _B. subtilis_ spores with ionizing radiation and revive spores synchronously under unperturbed conditions. In the absence of both end resection pathways (i.e., in the _\(\Delta\)recl_ _AaddAB_ strain), those reviving spores remain recombination proficient and are apparently as capable of repairing DNA damages as the wild-type (wt) control [17], suggesting that reviving spores activate other repair mechanisms in response to ionizing radiation treatment prior revival. Indeed, the ionizing radiation-induced broken ends can be reconnected by the Ku and LigD proteins via non-homologous end-joining [18], and the nicks can be repaired via a LigD-dependent pathway [19] during the ripening stage of spore revival. Later, at the early stage of outgrowth, the expression of _ku_ and _ligD_ is turned off, and that of DNA replication and repair proteins involved in single-strand gap repair and DDT is turned on [15; 16]. Here, replication fork impediments (damaged template bases, DNA distortions, replication-transcription conflicts) can be circumvented or bypassed by the action of RecA accessory proteins (e.g., RecO, RecR, RecF, RecX), RecA itself, the branch migration translocae RecG and the DNA damage checkpoint protein DisA, among others [17; 20]. The rationale is that, as a global response to replication stress in the absence of long-range end resection functions and an intact homologous template [15; 16; 17], as it occurs during the first replication cycle of reviving spores, it is necessary to use a different DDT mechanism (fork reversal, template switching, translesion synthesis) to avoid uncontrolled spore revival, that otherwise could be deleterious. (Unless stated otherwise, indicated genes and products are of _B.