The conserved SEN1 DNA/RNA helicase has multiple functions during yeast meiosis

Diploid Saccharomyces cerevisiae cells undergo meiosis when they are starved of nitrogen in the presence of a non-fermentable carbon source. Nutrient starvation triggers expression of Ime1, a master regulatory protein required to activate transcription of meiotic “early genes” that mediate premeiotic S phase and Prophase I processes, including recombination and chromosome synapsis. During prophase, the highly conserved, topisomerase-like protein, Spo11, generates ~200 double strand breaks that are used to identify homologous chromosomes and generate crossovers between them. DNA-RNA hybrids are formed when an RNA molecule anneals to a complementary strand of DNA and are present at the ends of double strand breaks during Prophase I of meiosis in a variety of organisms. One way of removing DNA/RNA hybrids is degradation of the RNA by RNase H. Phenotypic characterization of organisms lacking RNase H activity has demonstrated that regulation of the level of DNA-RNA hybrids is important for meiotic double strand break repair. Sen1 is an essential 5’-3’ DNA-RNA helicase that can remove DNA-RNA hybrids by unwinding the RNA. Sen1 is orthologous to the mammalian Senataxin (SETX) helicase. Mouse mutants lacking either Senataxin or RNase H activity exhibit male infertility and defects in double strand break repair. SETX is also required for meiotic sex chromosome inactivation, making it unclear whether SETX’s role in meiotic recombination is direct or an indirect consequence due to defects in SETX functions that affect transcription. Using a variety of orthogonal approaches, this work demonstrates that SEN1 has multiple, temporally distinct functions that promote yeast meiosis. First, it enables the timely expression of IME1-regulated early genes. Second, it helps prevent/remove DNA-RNA hybrids that form during premeiotic S phase. Third, it facilitates repair of Spo11 double strand breaks generated during Prophase I, as well as chromosome synapsis.二倍体酿酒酵母（Saccharomyces cerevisiae）在非发酵碳源存在且遭遇氮饥饿时，会启动减数分裂过程。营养饥饿会触发主调控蛋白Ime1的表达，该蛋白是激活减数分裂"早期基因"转录的必需因子，这些早期基因介导减数分裂前S期以及减数分裂I前期的相关过程，包括重组与染色体联会。在减数分裂I前期，高度保守的类拓扑异构酶蛋白Spo11会产生约200个双链断裂，这些断裂可用于识别同源染色体并介导其间的交叉互换。当RNA分子与DNA互补链退火结合时，便会形成DNA-RNA杂交体；在多种生物的减数分裂I前期，这类杂交体存在于双链断裂的末端区域。清除DNA/RNA杂交体的途径之一，是通过核糖核酸酶H（RNase H）降解其中的RNA。对缺失RNase H活性的生物体进行表型表征的研究表明，调控DNA-RNA杂交体的水平对于减数分裂双链断裂修复至关重要。Sen1是一种必需的5’-3’方向DNA-RNA解旋酶，可通过解旋RNA来清除DNA-RNA杂交体。Sen1与哺乳动物中的Senataxin（SETX）解旋酶为直系同源蛋白。缺失Senataxin或RNase H活性的小鼠突变体表现出雄性不育以及双链断裂修复缺陷。SETX同时也是减数分裂性染色体失活所必需的，这使得目前尚不清楚SETX在减数分裂重组中的作用是直接的，还是由其影响转录的功能缺陷所导致的间接结果。本研究通过多种正交实验方法证实，SEN1基因在酵母减数分裂过程中发挥多种时序特异性的功能：其一，它能够确保受IME1调控的早期基因得以按时表达；其二，它有助于预防并清除减数分裂前S期形成的DNA-RNA杂交体；其三，它能够促进减数分裂I前期产生的Spo11介导的双链断裂修复以及染色体联会过程。