Department of Microbiology, Boston University School of Medicine, Boston, MA, USA.
J Virol. 2013 Mar;87(6):3196-207. doi: 10.1128/JVI.02862-12. Epub 2013 Jan 2.
The mechanisms by which the respiratory syncytial virus (RSV) RNA-dependent RNA polymerase (RdRp) initiates mRNA transcription and RNA replication are poorly understood. A previous study, using an RSV minigenome, suggested that the leader (Le) promoter region at the 3' end of the genome has two initiation sites, one at position +1, opposite the 3' terminal nucleotide of the genome, and a second site at position +3, at a sequence that closely resembles the gene start (GS) signal of the RSV L gene. In this study, we show that the +3 initiation site of the Le is utilized with apparently high frequency in RSV-infected cells and yields small RNA transcripts that are heterogeneous in length but mostly approximately 25 nucleotides (nt) long. Experiments with an in vitro assay in which RSV RNA synthesis was reconstituted using purified RdRp and an RNA oligonucleotide showed that nt 1 to 14 of the Le promoter were sufficient to signal initiation from +3 and that the RdRp could access the +3 initiation site without prior initiation at +1. In a minigenome assay, nucleotide substitutions within the Le to increase its similarity to a GS signal resulted in more-efficient elongation of the RNA initiated from position +3 and a reduction in RNA initiated from the NS1 gene start signal at +45. Taken together, these data suggest a new model for initiation of sequential transcription of the RSV genes, whereby the RdRp initiates the process from a gene start-like sequence at position +3 of the Le.
呼吸道合胞病毒 (RSV) RNA 依赖性 RNA 聚合酶 (RdRp) 启动 mRNA 转录和 RNA 复制的机制尚未完全阐明。先前的一项研究使用 RSV 小基因组表明,基因组 3' 端的先导 (Le) 启动子区域有两个起始位点,一个位于位置 +1,与基因组的 3' 末端核苷酸相对,另一个位于位置 +3,序列与 RSV L 基因的基因起始 (GS) 信号非常相似。在这项研究中,我们表明,Le 的 +3 起始位点在 RSV 感染的细胞中被高频利用,并产生长度不均但主要约 25 个核苷酸 (nt) 长的小 RNA 转录物。使用体外测定的实验表明,使用纯化的 RdRp 和 RNA 寡核苷酸重新合成 RSV RNA,Le 启动子的 nt 1 到 14 足以发出从 +3 起始的信号,并且 RdRp 可以在没有预先从 +1 起始的情况下访问 +3 起始位点。在小基因组测定中,Le 中的核苷酸取代以增加其与 GS 信号的相似性,导致从 +3 起始的 RNA 的延伸更有效,并且从 NS1 基因起始信号 +45 起始的 RNA 减少。总之,这些数据表明了 RSV 基因顺序转录起始的新模型,其中 RdRp 从 Le 的 +3 位置的基因起始样序列开始启动该过程。