Pre-snRNA is elongated and capped with 7-methylguanosine

A 7-methylguanosine triphosphate group is added to the 5' end of the pre-snRNA during transcription elongation (Mattaj 1986). The capping enzyme and cap methyltransferase involved in mRNA capping may also be responsible for this reaction. In the case of mRNA capping, the capping enzyme is targeted to the pre-mRNA by interaction with the phosphorylated C-terminal domain (CTD) of RNA polymerase II (McCracken et al. 1997). During elongation, the phosphorylation pattern of the CTD also changes: serine-5 is dephosphorylated by RPAP2 (Egloff et al. 2012) interacting with RPRD1A and RPRD1B (Ni et al. 2011, Ni et al. 2014) and serine-2 is phosphoryated by P-TEFb. Serine-7 is also phosphorylated, possibly, however the responsible kinase is not certain. The order of the capping and phosphorylation events is unknown.

在转录延伸过程中，7-甲基鸟苷三磷酸基团被添加至前体信使RNA（snRNA）的5'端（参见Mattaj，1986年）。参与信使RNA帽合成的加帽酶和帽甲基转移酶可能也负责该反应。在信使RNA帽合成的情形中，加帽酶通过与RNA聚合酶II的磷酸化C端结构域（CTD）相互作用而被靶向至前体mRNA（参见McCracken等人，1997年）。在延伸过程中，CTD的磷酸化模式亦发生改变：丝氨酸-5通过RPAP2（Egloff等人，2012年）与RPRD1A和RPRD1B（Ni等人，2011年，Ni等人，2014年）的相互作用而被去磷酸化，而丝氨酸-2则被P-TEFb磷酸化。丝氨酸-7也可能被磷酸化，然而负责的激酶尚不明确。加帽和磷酸化事件的顺序尚属未知。