The dual nature of homologous recombination in plants

174Review TRENDS in Genetics Vol.21 No.3 March 2005Table 1. Homologous recombination genes in Arabidopsis, yeast and mammals aArabidopsis Refs Proposed function b Budding yeast MammalsATM [29] Double-strand break (DSB) signaling, cell-cycleTel1ATMcheckpointATR [72] DSB signaling, cell-cycle checkpoint Mec1 ATRBRCA2 c [22] Strand invasion – BRCA2BRU [35] Chromatin state, silencing ? ?Centrin2 [46] Nucleotide excision repair (NER) ? Centrin2DMC1 [20] Meiotic interstrand invasion Dmc1 DMC1? DSB repair? – DNA-PKERCC1 [73] NER Rad10 ERCC1INO80 [47] DSB repair, homologous recombination (HR) Ino80 INO80MIM [34,74] DNA repair and recombination Smc6? SMC6?MLH1 [26] Promotion of crossovers in meiosis Mlh1 MLH1MRE11 [17] DSB end processing Mre11 MRE11MSH4 [27] Promotion and stabilization of meiotic HR Msh4 MSH4? DNA damage signaling? – p53? DSB end processing? Xrs2 NBS1RAD1 [75–78] NER, nonhomologous overhang end removal Rad1 XPFRAD9 [39] Cell-cycle checkpoint Rad9 RAD9RAD17 [39] Cell-cycle checkpoint Rad17 RAD17RAD25 (also known as [79] NER Rad25 XPBXPB1)RAD50 [18,19] DSB end processing Rad50 RAD50RAD51 [21] Strand invasion Rad51 RAD51RAD51C [23] Holliday junction (HJ) resolution? – RAD51C? HR? Rad52 RAD52RAD54L? HR, perhaps through chromatin events? Rad54 RAD54SNM1 [44] Interstrand crosslink repair, recombination Snm1? SNM1?SPO11-1 [14,16] Induction of meiotic DSB Spo11 SPO11SPO11-2 [14,15] ? – –SPO11-3 [15] ? – –XRCC3 [22] HJ resolution? Rad57 XRCC3a The main factors involved in or potentially affecting somatic and meiotic HR are listed in alphabetical order with their function. Known homologs in budding yeast andmammals are shown. Yeast or mammalian genes for which no obvious homologs exist in Arabidopsis are highlighted in bold.b Refers to function in Arabidopsis.c Two homologs in Arabidopsis.the archaebacterial topoisomerase VI complex, whichinduces transient DSBs to disentangle DNA. In contrastto most other organisms, Arabidopsis possesses threeparalogs of Spo11 and a homolog of the topoisomerase VIBsubunit (TOP6B) [14,15] (Table 1). In an Arabidopsismutant of the SPO11-1 gene the formation of chiasmataand bivalents was severely reduced, and synapsis couldnot be observed [14,16]. Meiotic division proceeded in thespo11-1 background, resulting in ‘polyads’ of random DNAcontent instead of the typical tetrads. It remains to beelucidated whether production of residual DSBs and thereduced formation of chiasmata are due to alternativepathways or a result of the function of the two otherSPO11 paralogs. However, both of them but not SPO11-1were shown to interact with the homologs of the B subunit,suggesting a eukaryotic topoisomerase-like function [15].The budding yeast Mre11–Rad50–Nbs1 complex isassumed to participate in the Spo11-dependent inductionof DSBs and in the processing of the ends generating3 0 -protruding single stranded DNA (ssDNA). Mutantsof the Arabidopsis orthologs of Mre11 [17] and Rad50[18,19] were isolated and analyzed for their meioticphenotypes. Both mutants exhibited chromosomal fragmentationand failure of synapsis, supporting a rolefor the Arabidopsis MRE11–RAD50 complex in earlysteps of meiotic HR, presumably in processing the DNAends. In contrast to the situation in budding yeast, theArabidopsis protein complex is not required for inductionof the SPO11-mediated DSBs, because the chromosomefragmentation and the complete sterility of mre11 could bepartially suppressed in a spo11-1 background [17].In the subsequent step of yeast mHR, Rad51 (the homologof the bacterial RecA recombinase) and its meiotic paralogDmc1 assemble on the ssDNA to form a nucleoproteincomplex. This complex searches for homologous sequencesand finally triggers strand invasion to form a jointmolecule between parental chromosomes, a prerequisitefor synapsis. Consistent with this, Arabidopsis mutants inthe DMC1 [20] and RAD51 [21] genes were severelyaffected in mHR. Their chromosomes clearly failed tosynapse and to form bivalents, but unlike in yeast,chromosome fragmentation caused by persisting meioticDSBs was observed only in rad51 mutants. This suggestsa function of DMC1 in the selective invasion of thehomologous chromosome rather than in the repair ofthe SPO11-induced DSBs. These could be repaired byRAD51-mediated HR between sister chromatids or by analternative pathway. Interestingly, a DMC1 homolog couldnot be identified in Drosophila melanogaster nor inCaenorhabditis elegans, in both of which the synaptonemalcomplex is formed independently of SPO11-inducedDSBs [11]. Recently, the function of the two Arabidopsisrecombinases RAD51 and DMC1 was suggested to bedependent on their genetic and molecular interaction withthe two paralogs of BRCA2. The absence of synapsis andthe chromosome fragmentation phenotype in RNAi-brca2plants was reminiscent of that of the double-mutantdmc1/RNAi-rad51 [22], and the BRCA2 paralogswww.sciencedirect.com