RecQ helicases: lessons from model organisms

Abstract

RecQ DNA helicases function during DNA replication and are essential for the maintenance of genome stability. There is increasing evidence that spontaneous genomic instability occurs primarily during DNA replication, and that proteins involved in the S-phase checkpoint are a principal defence against such instability. Cells that lack functional RecQ helicases exhibit phenotypes consistent with an inability to fully resume replication fork progress after encountering DNA damage or fork arrest. In this review we will concentrate on the various functions of RecQ helicases during S phase in model organisms.

Figure 1

Members of the RecQ family of DNA helicases from E.coli, S.cerevisiae, S.pombe, C.elegans, X.laevis and H.sapiens. The size of each protein in amino acids is shown on the right and the regions corresponding to the helicase domain, and conserved regions RQC and HRDC are indicated and shown in the key below the figure. The NLS and exonuclease regions unique to the mammalian orthologs are also indicated.

Figure 2

During an HU block the Sgs1–Top3 complex functions to maintain DNA polymerase association at replication forks. (A) In wild-type cells the polymerases are maintained at stalled forks. (B) In the absence of Sgs1 helicase activity both DNA polymerase α and ɛ association with stalled forks is defective. Sgs1–Top3 could function to prevent or resolve the ‘chickenfoot’ structure (C) and/or favor DNA polymerase α and ɛ association through interaction with RPA (D), allowing for the resumption of DNA replication after the block is removed (E).

Figure 3

Pathways where Sgs1–Top3 function to process collapsed or stalled replication forks. (A) When a replication fork approaches a lesion (red circle) in the DNA a single-stranded gap forms and replication fork collapse may occur. This allows Rad51-dependent D-loop formation to initiate a recombination event via Holliday Junction (HJ) formation. The HJ may be processed in three ways. [1] The invading strand can be disrupted by the Srs2 helicase as part of the SDSA pathway leading to gene conversion without crossover events. [2] The Sgs1–Top3 complex can work in a dissolution pathway, where Sgs1 first promotes branch migration on the formed dHJ substrate. The single-stranded interlinked DNA is subsequent decatenated by Top3 resulting in gene conversion without crossover events. [3] Finally endonucleases such as Mus81–Mms4 may process the dHJ by two subsequent cleavage reactions. This will generate gene conversion products either with, or without an associated crossover event. Note that in the absence of Rrm3 more stalled replication forks will be generated. (B) When replication forks converge and stall at the rDNA, Sgs1–Top3 may allow DNA replication to finish by a DNA polymerase fill in reaction after separating the parental strands of the DNA template. In the absence of Sgs1–Top3, Slx1–Slx4 may cleave at each replication fork thereby creating a double nicked chromatid and a broken sister chromatid. The DSB can be repaired by a RAD52-independent single-strand annealing pathway at the rDNA.