Holliday junction resolvases
The formation of four-way DNA junctions, Holliday junctions, account for products formed during meiotic recombination. These junctions are involved recombinational repair of DNA double-stranded breaks. Resolvases are small homodimeric enzymes that resolve Holliday junctions by endonucleolytic cleavage. Resolution is achieved by the introduction of symmetrically related nicks in two strands of like polarity. In a recent study by Stephen Ip and colleagues, nucleases that promote Holliday junction resolution, in a similar manner to Escherichia coli Holliday junction resolvase RuvC, were identified in human cells and also the budding yeast Saccharomyces cerevisiae.
They stated that the identification of GEN1 and Yen1 Holliday junction resolvases suggests that the RuvC model is universal. GEN1 and Yen1 promote Holliday junction resolution by a mechanism analogous to that shown by RuvC, Cce1, Hjc and the bacteriophage resolvases, but they neither do show sequence nor structure conservation with these proteins, instead they belong to the Rad2/XPG family of nucleases. There are three known classes in this group. Class I (XPG in humans and Rad2 in yeast) — endonucleolytic incision at the 3′ side of bubble structures formed during nucleotide excision repair. Class II (FEN1 in humans and Rad27 in yeast) — 5′-flap endonuclease activities for Okazaki fragment processing during DNA replication. Class III — EXO1-related proteins which exhibit 5′–3′ exonuclease activities required for DNA replication, DNA repair and meiotic recombination.
The results described in their study indicate the eukaryotic Holliday junction resolvases comprise Class IV.
Nature. 2008 Nov 20;456(7220):357-61.
