Henriques Telmo, Ji Zhe, Tan-Wong Sue Mei, Carmo Alexandre M, Tian Bin, Proudfoot Nicholas J, Moreira Alexandra
Instituto de Biologia Molecular e Celular, Universidade do Porto, Porto, Portugal.
Transcription. 2012 Jul-Aug;3(4):198-212. doi: 10.4161/trns.21967.
Transcription termination of RNA polymerase II between closely spaced genes is an important, though poorly understood, mechanism. This is true, in particular, in the Drosophila genome, where approximately 52% of tandem genes are separated by less than 1 kb. We show that a set of Drosophila tandem genes has a negative correlation of gene expression and display several molecular marks indicative of promoter pausing. We find that an intergenic spacing of 168 bp is sufficient for efficient transcription termination between the polo-snap tandem gene pair, by a mechanism that is independent of Pcf11 and Xrn2. In contrast, analysis of a tandem gene pair containing a longer intergenic region reveals that termination occurs farther downstream of the poly(A) signal and is, in this case, dependent on Pcf11 and Xrn2. For polo-snap, displacement of poised polymerase from the snap promoter by depletion of the initiation factor TFIIB results in an increase of polo transcriptional read-through. This suggests that poised polymerase is necessary for transcription termination. Interestingly, we observe that polo forms a TFIIB dependent gene loop between its promoter and terminator regions. Furthermore, in a plasmid containing the polo-snap locus, deletion of the polo promoter causes an increase in snap expression, as does deletion of polo poly(A) signals. Taken together, our results indicate that polo forms a gene loop and polo transcription termination occurs by an Xrn2 and Pcf11 independent mechanism that requires TFIIB.
RNA聚合酶II在紧密相邻基因之间的转录终止是一种重要但尚未被充分理解的机制。在果蝇基因组中尤其如此，其中约52%的串联基因间隔小于1 kb。我们发现一组果蝇串联基因的基因表达呈负相关，并显示出几个指示启动子暂停的分子标记。我们发现，168 bp的基因间间隔足以使polo-snap串联基因对之间实现高效转录终止，其机制独立于Pcf11和Xrn2。相比之下，对一个基因间区域较长的串联基因对的分析表明，终止发生在聚腺苷酸化信号下游更远的位置，在这种情况下，依赖于Pcf11和Xrn2。对于polo-snap，通过消耗起始因子TFIIB使准备就绪的聚合酶从snap启动子上移位，会导致polo转录通读增加。这表明准备就绪的聚合酶对于转录终止是必需的。有趣的是，我们观察到polo在其启动子和终止子区域之间形成了一个依赖于TFIIB的基因环。此外，在一个包含polo-snap基因座的质粒中，删除polo启动子会导致snap表达增加，删除polo聚腺苷酸化信号也会如此。综上所述，我们的结果表明，polo形成了一个基因环，并且polo转录终止是通过一种独立于Xrn2和Pcf11且需要TFIIB的机制发生的。
