The BORDER family of negative transcription elongation factors regulates flowering time in Arabidopsis

背景回顾：Transcription initiation has long been considered a primary regulatory step in gene expression. Recent work, however, shows that downstream events, such as transcription elongation, can also play important roles. A well-characterized example from animals is promoter-proximal pausing, where transcriptionally engaged Pol II accumulates 30–50 bp downstream of the transcription start site (TSS) and is thought to enable rapid gene activation. Plants do not make widespread use of promoter-proximal pausing; however, in a phenomenon known as 3′ pausing, a significant increase in Pol II is observed near the transcript end site (TES) of many genes. 

研究基础：Previous work has shown that 3′ pausing is promoted by the BORDER (BDR) family of negative transcription elongation factors. 

主要研究：Here we show that BDR proteins play key roles in gene repression.

结果1-BDR-Pol II：Consistent with BDR proteins acting to slow or pause elongating Pol II, BDR-repressed genes are characterized by high levels of Pol II occupancy, yet low levels of mRNA. 

结果2-BDR-FPA-FLC：The BDR proteins physically interact with FPA, one of approximately two dozen genes collectively referred to as the autonomous floral-promotion pathway, which are necessary for the repression of the flowering time gene FLOWERING LOCUS C (FLC). 

结果3-FLC-H3K27me3-flower early：In early-flowering strains, FLC expression is repressed by repressive histone modifications, such as histone H3 lysine 27 trimethylation (H3K27me3), thereby allowing the plants to flower early. 

结论：These results suggest that the repression of transcription elongation by BDR proteins may allow for the temporary pausing of transcription or facilitate the long-term repression of genes by repressive histone modifications.

 摘 要 

长久以来，转录起始一直被认为是基因表达的主要调控阶段。但是，最近的一些研究显示，转录延伸等转录起始下游的一些事件在基因表达调控方面同样也发挥了非常重要的作用。来自动物的一个典型例子是启动子近端暂停，其中负责基因转录的RNA聚合酶(Pol) II在转录起始位点下游30-50bp处累积，被认为能够快速激活基因。植物中启动子近端暂停现象并不普遍；然而，植物中存在一种称为3′暂停的现象，即在许多基因的转录终止位点附近Pol II积累显著增加。先前的研究显示，植物中的3′暂停是由负转录延伸因子BDR家族所促进的。本文中，作者发现BDR蛋白在基因抑制方面发挥关键作用。与BDR蛋白物理减缓或暂停延伸的Pol II一致，受到BDR抑制的基因位点上往往积累了高水平的Pol II，但是mRNA水平仍然较低。BDR蛋白能够与成花自主途径中的一个蛋白FPA发生物理互作，这对于开花时间基因FLC的抑制是必须的。在早花品系中，FLC基因的表达受到H3K27me3等抑制性组蛋白修饰的抑制，从而保证植物能够提早开花。本文的研究结果表明BDR蛋白介导的转录延伸抑制可能会导致暂时的转录暂停，或通过抑制性组蛋白修饰促进基因的长期抑制。