## 1 Introduction
subtilis_ PcrA is essential for viability [36], neither _S. cerevisiae_ RRM3 nor _E. coli_ Rep are needed for viability [37; 38]. These enzymes are now considered to be accessory replicative helicases that minimise replisome pausing along protein-bound DNA, whilst the primary replicative helicase is responsible for template DNA unwinding and acts as a hub for replisome organisation [39; 40]. The above mechanisms reduce the probability of loss of function of replisomes encountering barriers that can be either cleared or bypassed. These mechanisms therefore rely on retention of function of paused replisomes. However, the large number of barriers encountered by replisomes means that there is still a significant risk of a replisome pausing at a barrier and losing function prior to bypass or clearance of the barrier [1; 7]. This is a particular problem with arrays of transcription complexes on highly transcribed genes [30; 41; 42; 43; 44; 45]. Blockage of a fork and loss of replisome function demands reloading of the replication machinery to complete genome duplication, even when multiple origins exist on the same chromosome [46]. Generation of a DNA structure onto which the replication machinery can be reloaded may require substantial remodelling of the fork DNA by a combination of exonucleases, endonucleases and helicases to facilitate replisome reloading [2; 7]. Such processing may also require strand exchange proteins either to reintegrate double-stranded DNA ends generated by fork processing, to repair single-stranded DNA gaps or to catalyse replication fork regression [1; 47; 48]. Strand exchange proteins might also promote blocked fork stabilisation, inhibiting extensive degradation of nascent DNA via occlusion of nucleases [49; 50; 51]. The bacterial strand exchange protein RecA minimises degradation of nascent DNA in _E. coli_ cells exposed to UV light [52]. This minimisation also requires RecFOR, factors that promote RecA loading onto ssDNA gaps rather than dsDNA ends, together with RecJ exonuclease and RecQ helicase [52; 53]. The general view now is that a major role of recombination enzymes, if not their primary purpose, is to underpin replication fork movement [54]. The importance of such enzymes is illustrated by the extensive DNA degradation in _recA_ mutant cells [55]. This degradation is catalysed by RecBCD, a helicase and exonuclease that unwinds and degrades dsDNA ends [55; 56]. Degradation of both DNA strands by RecBCD is rapid and processive but recognition of a specific DNA sequence, a x site, within the DNA inhibits degradation of the 3' ended strand and promotes loading of RecA onto this strand [56]. However, degradation continues in the absence of RecA, with RecBCD being able to degrade an entire chromosome arm [55; 57]. Some blocked forks may also undergo regression and extrude a dsDNA arm which may be degraded by RecBCD in the absence of RecA, effectively destroying the extruded arm of the fork and regenerating a fork structure onto which the replisome can be reloaded [58]. Targeting of blocked forks by recombination enzymes comes at the cost of genome rearrangements [59; 60]. This genetic instability is a particular problem at highly transcribed genes due to the density of transcribing RNA polymerases and the consequent high probability of fork pausing, loss of replisome function and the need to process the DNA via recombination enzymes to reload the replisome [2; 61; 62; 63]. Moreover, loss of factors that minimise stalled and backtracked transcription complexes increase the dependence of _E. coli_ cells on recombination enzymes [42]. The absence of accessory replicative helicases that restart paused forks also exacerbates the pathological effects of replication-associated recombination [64; 65]. Thus, _E. coli_ Rep limits harmful RecA loading at blocked forks [64]. Increasing the probability of fork pausing or of paused forks losing function therefore results in an increased need for recombination enzymes to underpin genome duplication.