Inhibiting translation elongation can aid genome duplication in Escherichia coli.

Conflicts between replication and transcription challenge chromosome duplication. Escherichia coli replisome movement along transcribed DNA is promoted by Rep and UvrD accessory helicases with Δrep ΔuvrD cells being inviable under rapid growth conditions. We have discovered that mutations in a tRNA gene, aspT, in an aminoacyl tRNA synthetase, AspRS, and in a translation factor needed for efficient proline-proline bond formation, EF-P, suppress Δrep ΔuvrD lethality. Thus replication-transcription conflicts can be alleviated by the partial sacrifice of a mechanism that reduces replicative barriers, namely translating ribosomes that reduce RNA polymerase backtracking. Suppression depends on RelA-directed synthesis of (p)ppGpp, a signalling molecule that reduces replication-transcription conflicts, with RelA activation requiring ribosomal pausing. Levels of (p)ppGpp in these suppressors also correlate inversely with the need for Rho activity, an RNA translocase that can bind to emerging transcripts and displace transcription complexes. These data illustrate the fine balance between different mechanisms in facilitating gene expression and genome duplication and demonstrate that accessory helicases are a major determinant of this balance. This balance is also critical for other aspects of bacterial survival: the mutations identified here increase persistence indicating that similar mutations could arise in naturally occurring bacterial populations facing antibiotic challenge.

Rep provides a second motor at the replisome to promote duplication of protein-bound DNA.

Nucleoprotein complexes present challenges to genome stability by acting as potent blocks to replication. One attractive model of how such conflicts are resolved is direct targeting of blocked forks by helicases with the ability to displace the blocking protein-DNA complex. We show that Rep and UvrD each promote movement of E. coli replisomes blocked by nucleoprotein complexes in vitro, that such an activity is required to clear protein blocks (primarily transcription complexes) in vivo, and that a polarity of translocation opposite that of the replicative helicase is critical for this activity. However, these two helicases are not equivalent. Rep but not UvrD interacts physically and functionally with the replicative helicase. In contrast, UvrD likely provides a general means of protein-DNA complex turnover during replication, repair, and recombination. Rep and UvrD therefore provide two contrasting solutions as to how organisms may promote replication of protein-bound DNA.

Protein-DNA complexes are the primary sources of replication fork pausing in Escherichia coli.

Replication fork pausing drives genome instability, because any loss of paused replisome activity creates a requirement for reloading of the replication machinery, a potentially mutagenic process. Despite this importance, the relative contributions to fork pausing of different replicative barriers remain unknown. We show here that Deinococcus radiodurans RecD2 helicase inactivates Escherichia coli replisomes that are paused but still functional in vitro, preventing continued fork movement upon barrier removal or bypass, but does not inactivate elongating forks. Using RecD2 to probe replisome pausing in vivo, we demonstrate that most pausing events do not lead to replisome inactivation, that transcription complexes are the primary sources of this pausing, and that an accessory replicative helicase is critical for minimizing the frequency and/or duration of replisome pauses. These findings reveal the hidden potential for replisome inactivation, and hence genome instability, inside cells. They also demonstrate that efficient chromosome duplication requires mechanisms that aid resumption of replication by paused replisomes, especially those halted by protein-DNA barriers such as transcription complexes.

Interaction of Rep and DnaB on DNA.

Genome duplication requires not only unwinding of the template but also the displacement of proteins bound to the template, a function performed by replicative helicases located at the fork. However, accessory helicases are also needed since the replicative helicase stalls occasionally at nucleoprotein complexes. In Escherichia coli, the primary and accessory helicases DnaB and Rep translocate along the lagging and leading strand templates, respectively, interact physically and also display cooperativity in the unwinding of model forked DNA substrates. We demonstrate here that this cooperativity is displayed only by Rep and not by other tested helicases. ssDNA must be exposed on the leading strand template to elicit this cooperativity, indicating that forks blocked at protein-DNA complexes contain ssDNA ahead of the leading strand polymerase. However, stable Rep-DnaB complexes can form on linear as well as branched DNA, indicating that Rep has the capacity to interact with ssDNA on either the leading or the lagging strand template at forks. Inhibition of Rep binding to the lagging strand template by competition with SSB might therefore be critical in targeting accessory helicases to the leading strand template, indicating an important role for replisome architecture in promoting accessory helicase function at blocked replisomes.

Localization of an accessory helicase at the replisome is critical in sustaining efficient genome duplication.

Genome duplication requires accessory helicases to displace proteins ahead of advancing replication forks. Escherichia coli contains three helicases, Rep, UvrD and DinG, that might promote replication of protein-bound DNA. One of these helicases, Rep, also interacts with the replicative helicase DnaB. We demonstrate that Rep is the only putative accessory helicase whose absence results in an increased chromosome duplication time. We show also that the interaction between Rep and DnaB is required for Rep to maintain rapid genome duplication. Furthermore, this Rep-DnaB interaction is critical in minimizing the need for both recombinational processing of blocked replication forks and replisome reassembly, indicating that colocalization of Rep and DnaB minimizes stalling and subsequent inactivation of replication forks. These data indicate that E. coli contains only one helicase that acts as an accessory motor at the fork in wild-type cells, that such an activity is critical for the maintenance of rapid genome duplication and that colocalization with the replisome is crucial for this function. Given that the only other characterized accessory motor, Saccharomyces cerevisiae Rrm3p, associates physically with the replisome, our demonstration of the functional importance of such an association indicates that colocalization may be a conserved feature of accessory replicative motors.

DNA structure specificity conferred on a replicative helicase by its loader.

Prokaryotic and eukaryotic replicative helicases can translocate along single-stranded and double-stranded DNA, with the central cavity of these multimeric ring helicases being able to accommodate both forms of DNA. Translocation by such helicases along single-stranded DNA results in the unwinding of forked DNA by steric exclusion and appears critical in unwinding of parental strands at the replication fork, whereas translocation over double-stranded DNA has no well-defined role. We have found that the accessory factor, DnaC, that promotes loading of the Escherichia coli replicative helicase DnaB onto single-stranded DNA may also act to confer DNA structure specificity on DnaB helicase. When present in excess, DnaC inhibits DnaB translocation over double-stranded DNA but not over single-stranded DNA. Inhibition of DnaB translocation over double-stranded DNA requires the ATP-bound form of DnaC, and this inhibition is relieved during translocation over single-stranded DNA indicating that stimulation of DnaC ATPase is responsible for this DNA structure specificity. These findings demonstrate that DnaC may provide the DNA structure specificity lacking in DnaB, limiting DnaB translocation to bona fide replication forks. The ability of other replicative helicases to translocate along single-stranded and double-stranded DNA raises the possibility that analogous regulatory mechanisms exist in other organisms.