PHS1 regulates meiotic recombination and homologous chromosome pairing by controlling the transport of RAD50 to the nucleus

Abstract

Recombination and pairing of homologous chromosomes are critical for bivalent formation in meiotic prophase. In many organisms, including yeast, mammals, and plants, pairing and recombination are intimately interconnected. The POOR HOMOLOGOUS SYNAP-SIS1 (PHS1) gene acts in coordination of chromosome pairing and early recombination steps in plants, ensuring pairing fidelity and proper repair of meiotic DNA double-strand-breaks. In phs1 mutants, chromosomes exhibit early recombination defects and frequently associate with non-homologous partners, instead of pairing with their proper homologs. Here, we show that the product of the PHS1 gene is a cytoplasmic protein that functions by controlling transport of RAD50 from cytoplasm to the nucleus. RAD50 is a component of the MRN protein complex that processes meiotic double-strand-breaks to produce single-stranded DNA ends, which act in the homology search and recombination. We demonstrate that PHS1 plays the same role in homologous pairing in both Arabidopsis and maize, whose genomes differ dramatically in size and repetitive element content. This suggests that PHS1 affects pairing of the gene-rich fraction of the genome rather than preventing pairing between repetitive DNA elements. We propose that PHS1 is part of a system that regulates the progression of meiotic prophase by controlling entry of meiotic proteins into the nucleus. We also document that in phs1 mutants in Arabidopsis, centromeres interact before pairing commences along chromosome arms. Centromere coupling was previously observed in yeast and polyploid wheat while our data suggest that it may be a more common feature of meiosis.maize ͉ meiosis ͉ recombination ͉ chromosome pairing M eiotic prophase I encompasses a large number of tightly regulated and coordinated processes (1). Although many structural proteins involved in these processes are now known, mechanisms regulating meiotic prophase progression remain largely unexplored. In this study, we describe the function of the POOR HOMOLOGOUS SYNAPSIS (PHS1) gene in higher plants that regulates prophase I by coordinating recombination and homologous chromosome pairing (2). Meiotic recombination is initiated by SPO11-mediated formation of double-strand breaks (DSBs) in chromosomal DNA (3). In plants, this occurs in or before early leptotene (2, 4). The DSBs are subsequently resected by the MRN protein complex in plants and animals, and a similar MRX complex in S. cerevisiae, to generate singlestranded DNA (ssDNA) overhangs. The MRN complex contains MRE11, RAD50, and NBS1 (5, 6). MRE11 is a DNA-binding protein with endonuclease, exonuclease, and helicase activities that directly facilitate ssDNA overhang formation (6, 7). RAD50 most likely plays a structural role in the complex (6,8). NBS1 regulates the activity of MRE11 and signals DSB presence (5, 6). ssDNA tails created by DSB resection are coated by two DNA strand-exchange proteins, RAD51 and DMC1 (9). The nucleoprotein filaments formed in this way find and invade doublestranded DNA regions on the homo...

Chromosome Organization and Dynamics during Interphase, Mitosis, and Meiosis in Plants

“…Studies in a variety of eukaryotes, including plant model species Arabidopsis and maize, suggest that successful completion of homologous chromosome pairing is tightly linked to the progression of meiotic recombination (Franklin et al, 1999;Li et al, 2004;Pawlowski et al, 2004;Ronceret et al, 2009). This intimate dependence of pairing on recombination exists also in fungi and mammals (Baudat et al, 2000;Peoples-Holst and Burgess, 2005), but, interestingly, not in Drosophila melanogaster or Caenorhabditis elegans (Dernburg et al, 1998;McKim et al, 1998).…”

Recognition and Repair Pathways of Damaged DNA in Higher Plants

“…Several alternative pathways may exist that allow HR based repair of DSBs, however, good evidence is provided for at least two alternative pathways in plants. One is the synthesis-dependent strand annealing (SDSA) mechanism which involves the meiotic recombination11/Rad50/X-ray sensitive 2 (MRN) complex (Waterworth et al, 2007;Ronceret et al, 2009;Amiard et al, 2010). The MRN complex is discussed to function as a first sensor of double strand breaks.…”

The effect of different genome and cytoplasm on meiotic pairing in maize newly synthetic polyploids

“…In addition, some other reports find that genome instability of arabidopsis is controlled by both genetic and environmental factors (Armstrong et al 2001). Poor homologous synapsis 1 (Phs1) and penetration and arbuscule morphogenesis 1 (Pam1) also play an important role in genome instability (Ronceret et al 2009;Lukaszewski and Kopecky 2010;Feddermann et al 2010). As a whole, the genome instability in the early stage of polyploidization is affected by multiple factors, in order to have a further understanding, more experimental models and more experimental methods should be explored.…”