## 1 Introduction
Such is the potentially catastrophic effect of unregulated strand exchange that organisms have also evolved other means of limiting binding of strand exchange proteins to ssDNA. Turnover of strand exchange protein-ssDNA filaments by helicases is a key mechanism employed in both bacteria and eukaryotes to limit homologous recombination [66] with UvrD helicase performing this task in _E. coli_[67]. Accessory helicases target paused, active replisomes whereas recombination enzymes process blocked forks that no longer retain an active replisome. The substrates for these classes of enzymes are therefore very different. _S. cerevisiae rrm3_ mutant cells are viable but require replication, repair and checkpoint genes for normal growth [68; 69; 70]. Similarly, _E. coli_ cells lacking either RecBCD or Rep are viable but cells lacking both are inviable [71; 72]. In contrast, the viability of _E. coli recA rep_ mutant cells indicates that processing of inactivated forks does not necessarily require strand exchange [71]. This viability reflects the ability of RecBCD to degrade partly replicated chromosomes when RecA is absent [55; 58]. Indeed, RecBCD but not RecA, is essential for viability in the presence of an inverted and highly expressed ribosomal operon [73]. It should be borne in mind, though, that RecBCD activity in the presence of RecA results in loading of RecA onto the single-stranded DNA generated by RecBCD, strand exchange and priming of DNA replication via a D-loop recombination intermediate [7; 56; 74]. _E. coli_\(\Delta\)_rep_ mutant cells are viable in part because a homologous helicase, UvrD, can also promote fork movement along protein-bound DNA and, thus, compensate partially for the absence of Rep [22; 30]. Single deletion mutants are therefore viable whereas \(\Delta\)_rep_\(\Delta\)_ucrrD_ mutants are not [75]. The lack of full compensation may be because UvrD, unlike Rep, does not interact with the replisome via DnaB [22]. DinG helicase has also been implicated in resolving conflicts between replication and transcription in concert with Rep and/or UvrD [30]. However, the mechanistic basis of this interplay remains unclear with no direct evidence that DinG displaces proteins ahead of advancing replication forks [40]. It is clear, though, that \(\Delta\)_rep_ mutant cells but neither \(\Delta\)_ucrrD_ nor \(\Delta\)_dinG_ mutants exhibit a significant extension of the time needed to replicate the chromosome [29; 31]. This Rep-specific defect indicates that Rep rather than UvrD or DinG plays a key role in maintaining rapid fork movement. \(\Delta\)_rep_\(\Delta\)_ucrD_ double mutant inviability can be suppressed by growth of \(\Delta\)_rep_\(\Delta\)_ucrD_ mutant cells on minimal medium, conditions under which levels of transcription are reduced as compared with rich medium growth [22; 30]. _Arrep_\(\Delta\)_ucrD_ inviability on rich medium is also partially suppressed by two classes of mutation. One class of mutants harbour mutations in _spoT_ which leads to elevated concentrations of the signalling molecule (p)ppGpp [22]. (p)ppGpp binds to RNA polymerase and inhibits initiation of transcription of many genes including the _rrn_ operons in _E. coli_, the source of half of all transcription under rapid growth conditions, and also destabilises stalled transcription complexes [42; 76]. These effects may reduce the number of replicative barriers presented by transcription. Elevated (p)ppGpp also reduces replication elongation rates which might result in fewer collisions between transcription and replication, although the elongation rate is only modestly affected in _E. coli_[77]. The second class of mutations reside in the structural genes for RNA polymerase [22; 30; 78]. These mutations may suppress via different mechanisms depending on the nature of the mutation but may act in a similar manner to elevated (p)ppGpp [42; 78; 79] and/or reduce the extent of backtracking of paused RNA polymerases [80].