R-loop landscapes in yeast meiosis

Meiotic recombination is initiated by the Spo11-dependent programmed DNA double-strand breaks (DSBs) that are preferentially concentrated within genomic regions known as hotspots, but the factor(s) which specify the positions for meiotic DSB hotspots remain unclear. Here, we show that the frequency and distribution of R-loops, a type of functional chromatin structure comprising single-stranded DNA and a DNA:RNA hybrid, change dramatically throughout meiosis. We detected the formation of multiple de novo R-loops in the pachytene stage, and found they co-localize with meiotic DSB hotspots. We further show that transcription-replication head-on collisions could promote R-loop formation during meiosis, and apparently direct the initiation of meiotic DSB formation. Furthermore, the hotspots can be eliminated by reverse the direction of either transcription or replication, and reconstituted by reverse both of their direction. Our study reveals that R-loops may play dual roles in meiotic recombination. In addition to participation in meiotic DSB processing, some meiotic DSB hotspots may be originated from the transcription-replication head-on collisions during meiotic DNA replication.减数分裂重组由依赖Spo11的程序性DNA双链断裂（DSBs）启动，这类断裂优先富集于被称为热点的基因组区域，但目前仍未明确界定减数分裂DSB热点位置的具体调控因子。本研究发现，R环（R-loops）——一种由单链DNA与DNA:RNA杂交链构成的功能性染色质结构——的丰度与分布在减数分裂进程中发生显著改变。我们在粗线期检测到了多种新生R环的形成，并证实这些R环与减数分裂DSB热点存在共定位现象。进一步研究表明，转录-复制迎面碰撞可在减数分裂期间促进R环形成，并似乎直接介导了减数分裂DSB的起始过程。此外，通过反转转录或复制任一方向即可消除这类热点，而同时反转二者方向则可重建该类热点。本研究揭示了R环在减数分裂重组中可能兼具双重功能：除参与减数分裂DSB的加工过程外，部分减数分裂DSB热点或许起源于减数分裂DNA复制期间发生的转录-复制迎面碰撞。