Sowden M, Hamm J K, Spinelli S, Smith H C
Department of Biochemistry, University of Rochester School of Medicine and Dentistry, New York 14642, USA.
RNA. 1996 Mar;2(3):274-88.
Editing the apolipoprotein B (apoB) RNA involves deamination of cytidine by the catalytic subunit, APOBEC-1, as a component of an editosome. A tripartite sequence (editing motif) is essential for editosome assembly and site-specific editing. Current theory for the regulation of apoB RNA editing proposes that APOBEC-1 is rate limiting in cells and determines the proportion of edited apoB mRNAs. An evaluation of how the overexpression of APOBEC-1 increased the proportion of edited RNAs has led to the discovery of a paradox. McArdle cells edit a constant proportion of apoB RNA regardless of the total number of apoB RNAs expressed. Despite virtually identical editing motifs, apoB RNA transcripts from the endogenous gene and transfected, exogenous cDNA were edited with characteristic, but different efficiencies. This suggested that these RNAs were interacting with the editing machinery as distinct and noncompeting populations. We evaluated whether the presence of introns in the endogenous transcript may have distinguished it as a distinct population having reduced editing efficiency. The editing efficiency of chimeric splicing-editing RNA substrates was highest on fully processed RNA. Increased exon length improved utilization of the editing motif in these chimeric constructs. Taken together, the data suggest that the close proximity of introns can reduce apoB RNA editing efficiency. A population "gating" hypothesis is proposed wherein the proportion of edited RNAs in a population is determined my multiple cis- and trans-acting factors as RNAs pass through a nuclear restriction point.
载脂蛋白B（apoB）RNA的编辑涉及胞苷脱氨，由催化亚基APOBEC-1作为编辑体的一个组成部分来完成。一个三联体序列（编辑基序）对于编辑体组装和位点特异性编辑至关重要。目前关于apoB RNA编辑调控的理论认为，APOBEC-1在细胞中是限速的，并决定了编辑后的apoB mRNA的比例。对APOBEC-1过表达如何增加编辑后RNA比例的评估导致了一个矛盾现象的发现。无论表达的apoB RNA总数如何，McArdle细胞编辑的apoB RNA比例恒定。尽管编辑基序几乎相同，但来自内源基因和转染的外源cDNA的apoB RNA转录本以特征性但不同的效率进行编辑。这表明这些RNA作为不同且不相互竞争的群体与编辑机制相互作用。我们评估了内源转录本中内含子的存在是否可能将其区分为编辑效率降低的不同群体。嵌合剪接编辑RNA底物的编辑效率在完全加工的RNA上最高。外显子长度的增加提高了这些嵌合构建体中编辑基序的利用率。综合来看，数据表明内含子的紧密接近会降低apoB RNA的编辑效率。提出了一种群体“门控”假说，即当RNA通过核限制点时，群体中编辑后RNA的比例由多种顺式和反式作用因子决定。
