Replication fork reactivation downstream of a blocked nascent leading strand


Unrepaired lesions in the DNA template pose a threat to accurate replication. Several pathways exist in Escherichia coli to reactivate a blocked replication fork. The process of recombination-dependent restart of broken forks is well understood, but the consequence of replication through strand-specific lesions is less well known. Here we show that replication can be restarted and leading-strand synthesis re-initiated downstream of an unrepaired block to leading-strand progression, even when the 3′-OH of the nascent leading strand is unavailable. We demonstrate that the loading by a replication restart system of a single hexamer of the replication fork helicase, DnaB, on the lagging-strand template is sufficient to coordinate priming by the DnaG primase of both the leading and lagging strands. These observations provide a mechanism for damage bypass during fork reactivation, demonstrate how daughter-strand gaps are generated opposite leading-strand lesions during the replication of ultraviolet-light-irradiated DNA, and help to explain the remarkable speed at which even a heavily damaged DNA template is replicated.

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Figure 1: The leading strand can be reprimed during both PriA- and PriC-dependent restart.
Figure 2: A modified linear template in which the fork 3′-arm is replaced with a biotin group.
Figure 3: A single DnaB hexamer on the lagging-strand template coordinates priming of both strands.
Figure 4: PriC-dependent restart of a stalled fork generates daughter-strand gaps.


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These studies were supported by a grant from the NIH.

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Correspondence to Kenneth J. Marians.

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Heller, R., Marians, K. Replication fork reactivation downstream of a blocked nascent leading strand. Nature 439, 557–562 (2006).

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