Cleavage of the 3'-end of the Histone Pre-mRNA

Processing is initiated once the U7 snRNP is loaded onto the pre-mRNA. The pre-mRNA HDE makes base-pairing contacts with the 5”² end of U7 snRNA. Binding of the U7 snRNP to the pre-mRNA is stabilized by interactions between a U7 snRNP protein, hZFP100 and other trans-acting factors, including the factor that catalyzes the cleavage reaction, which have yet to be defined. The cleavage occurs in the presence of EDTA as does the cleavage reaction in polyadenylation, it is likely that this reaction is catalyzed by a protein. There may well be additional proteins associated with the U7 snRNP, since the in vitro processing occurs in the absence of SLBP, it is possible that all the other factors required for processing are associated with the active form of the U7 snRNP.

启动加工过程的前提是U7 snRNP被加载到前体mRNA上。前体mRNA中的HDE与U7 snRNA的5'²端形成碱基配对。U7 snRNP与前体mRNA的结合通过U7 snRNP蛋白、hZFP100及其他转录作用因子之间的相互作用得到稳定，包括催化切割反应的因子，该因子尚未被明确定义。切割反应在EDTA的存在下发生，与聚腺苷酸化过程中的切割反应相似，推测该反应可能由蛋白质催化。鉴于体外加工过程在缺乏SLBP的情况下进行，很可能与U7 snRNP相关的所有其他加工所需因子均与U7 snRNP的活性形式相联系。