The Regulation of Homologous Recombination by Helicases

Abstract

Homologous recombination is essential for DNA repair, replication and the exchange of genetic material between parental chromosomes during meiosis. The stages of recombination involve complex reorganization of DNA structures, and the successful completion of these steps is dependent on the activities of multiple helicase enzymes. Helicases of many different families coordinate the processing of broken DNA ends, and the subsequent formation and disassembly of the recombination intermediates that are necessary for template-based DNA repair. Loss of recombination-associated helicase activities can therefore lead to genomic instability, cell death and increased risk of tumor formation. The efficiency of recombination is also influenced by the 'anti-recombinase' effect of certain helicases, which can direct DNA breaks toward repair by other pathways. Other helicases regulate the crossover versus non-crossover outcomes of repair. The use of recombination is increased when replication forks and the transcription machinery collide, or encounter lesions in the DNA template. Successful completion of recombination in these situations is also regulated by helicases, allowing normal cell growth, and the maintenance of genomic integrity.

Keywords: DNA repair; anti-recombinase; helicase; recombination; replication; transcription.

Figure 1

Helicase Proteins involved in the generation of resected DNA ends during recombination. Multiple helicases, such as Bloom Syndrome helicase (BLM) and Sgs1, promote the formation of 3′ single-stranded DNA overhangs necessary for recombination. Other helicases, such as HelB, limit resection, or promote other pathways for repair. For full details, see text.

Figure 2

Displacement of RAD51 from resected DNA breaks by helicase proteins. The stability of the RAD51 nucleoprotein filament is regulated by several helicases, which can remove RAD51, thereby reducing the efficiency of recombination.

Figure 3

Helicase-mediated unwinding of displacement loop (D-loop) intermediates. Strand invasion of a broken DNA molecule into a homologous duplex creates a D-loop. Template-based repair of sequence at the break site can proceed at the paired 3′ end, using the homologous DNA as a template. This process is inhibited by the action of a number of helicases, such as Srs2 and RTEL1, which exhibit ‘anti-recombinase’ activity by unwinding the D-loop. Other helicases, such as HFM1 and MCM8-9, stabilize the D-loop by supporting DNA polymerase activity, increasing the amount of paired heteroduplex DNA.

Figure 4

Regulation of Holliday Junction disassembly by helicases. Double-Holliday junctions can be moved into close proximity by the branch migration activity of several helicase molecules. Protein complexes formed by helicases such as BLM and Sgs1 promote dissolution of the hemicatenane intermediate produced by branch migration, leading to non-crossover products of recombination.

Figure 5

Activity of helicases during DNA replication and transcription. Replication stress caused by a block in replisome progress at a DNA break or lesion can trigger replication fork reversal, which is regulated by a number of helicases. Newly synthesized DNA at the reversed fork is bound by RAD51, protecting it from nucleolytic degradation. Protection of nascent DNA is supported by the presence of several helicases.