Yu Kefei, Roy Deepankar, Bayramyan Melina, Haworth Ian S, Lieber Michael R
Department of Biochemistry and Molecular Biology, USC Norris Cancer Ctr., Rm. 5428, 1441 Eastlake Ave., MC9176, Los Angeles, CA 90033, USA.
Mol Cell Biol. 2005 Mar;25(5):1730-6. doi: 10.1128/MCB.25.5.1730-1736.2005.
Activation-induced deaminase (AID) is essential for class switch recombination and somatic hypermutation, and it has the ability to deaminate single-stranded DNA at cytidines. Mammalian class switch regions form R-loops upon transcription in the physiological orientation. The displaced DNA strand of an R-loop is forced to wrap around the RNA-DNA hybrid; hence, it may not have complete exposure to proteins. A fundamental question concerns the extent to which AID is accessible to the displaced strand of a transcription-generated R-loop. We used a minimal R-loop to carry out high-resolution analysis of the precise locations of AID action. We found that AID deaminates on the displaced DNA strand across the entire length of the R-loop. Displaced strand locations with a WRC (where W is A or T and R is A or G) sequence are preferred targets, but there are clear exceptions. These WRC deviations may be due to steric constraints on the accessibility of AID to these sites as the displaced strand twists around the RNA-DNA duplex. This phenomenon may explain the lack of WRC site preference at the mutations surrounding class switch recombination junctions.
活化诱导胞嘧啶脱氨酶（AID）对于类别转换重组和体细胞高频突变至关重要，它能够在胞嘧啶处使单链DNA发生脱氨基作用。哺乳动物的类别转换区域在以生理方向转录时会形成R环。R环中被置换的DNA链被迫缠绕在RNA-DNA杂交体周围；因此，它可能无法完全暴露于蛋白质。一个基本问题涉及AID可接触转录产生的R环的被置换链的程度。我们使用一个最小的R环对AID作用的精确位置进行高分辨率分析。我们发现AID在R环全长的被置换DNA链上进行脱氨基作用。具有WRC（其中W为A或T，R为A或G）序列的被置换链位置是优先靶点，但也有明显例外。这些WRC偏差可能是由于当被置换链围绕RNA-DNA双链体扭曲时，AID接近这些位点受到空间位阻限制。这种现象可能解释了在类别转换重组连接处周围的突变中缺乏WRC位点偏好的原因。
