## 1 Introduction  
The replication machineries of all organisms encounter many potential barriers whilst duplicating their genomes, presenting a major challenge to the maintenance of genetic stability [1; 2]. These barriers include damage to the template, non-B form DNA structures, topological strain and proteins bound to the DNA. Transcription provides both a topological challenge to DNA replication due to the over- and underwinding ahead of and behind an advancing RNA polymerase [3; 4] and substantial nucleoprotein barriers to fork movement due to their very high affinity [5]. Individual nucleoprotein complexes may have a low probability of halting a replication fork but the large number of barriers encountered creates a substantial risk of failure to complete high fidelity genome duplication [6; 7]. Replisomes paused at these barriers retain activity but this activity is lost as a function of time [8; 9; 10; 11]. There is thus a window of opportunity for removal or bypass of the barrier and resumption of replication by the paused replisome. If clearance or bypass of the barrier does not occur prior to loss of paused replisome function then the replication machinery must be reloaded back onto the chromosome to facilitate completion of genome duplication [1]. Given the importance of completing high fidelity genome duplication, all organisms have evolved mechanisms to underpin replisome movement by facilitating the restart of paused replisomes and by reconstituting an active replication fork after loss of paused replisome activity. Upon encountering a barrier, the replisome itself can clear or bypass certain types of obstacle. Forks paused at single-stranded DNA lesions may bypass the lesion by repriming replication downstream of the barrier, allowing resumption of replication at the cost of a gap in one of the nascent DNA strands [12; 13; 14; 15; 16]. However, bypass does not approach 100% efficiency, implying that replisomes encountering many lesions have a significant probability of losing activity [15]. Specialised translesion DNA polymerases can also replicate across such lesions under certain circumstances but often at the cost of errors in base incorporation [17; 18; 19]. The replisome is also capable of displacing proteins bound to the template DNA [20; 21; 22], a property that reflects the ability of helicases to disrupt the non-covalent bonding between proteins and DNA [23; 24]. Forks do also pause stochastically at protein-DNA complexes but the paused replisome may resume movement if the blocking protein dissociates from the DNA prior to loss of activity of the paused replisome [20]. However, the barriers posed by the many protein-DNA complexes found within a chromosome, especially those associated with transcription, appear to be too numerous and/or too long-lived for the replisome itself to deal with during the course of genome duplication. The _Saccharomyces cerevisiae_ RRM3 helicase minimises fork blockage at non-histone protein-DNA complexes and is required for normal rates of fork movement [25; 26; 27; 28]. Similarly, the _E. coli_ Rep helicase promotes fork movement through nucleoprotein complexes and its absence results in at least a twofold increase in the time needed to replicate a chromosome [22; 29; 30; 31]. This increase in the time needed for genome duplication reflects the function of Rep in minimising the frequency and/or duration of replisome pausing at protein-DNA complexes, the primary sources of replication pausing in _E. coli_[6]. The _Bacillus subtilis_ helicase PcrA, a homologue of _E. coli_ Rep, also facilitates replication of transcribed DNA in vivo [32], indicating conservation of this function across evolutionarily very divergent organisms. Both RRM3 and Rep also associate physically with components of their respective replisomes [22; 28; 33]. In _E. coli_ the physical association between Rep and the primary replicative helicase DnaB promotes cooperative DNA unwinding and nucleoprotein complex removal by the two helicases [22; 34; 35]. However, although _B.