Figure 2.

Maturation of the 7SK snRNP. The upper panels depict the maturation events according to the current model. The 7SK RNA is immediately bound by La (blue) upon transcription by Pol III. While the La module of La binds the 3ʹ end of the 7SK RNA, other regions of La might contact the 5ʹ end of bound substrates as well [147,148]. Binding of MePCE (green) to the 7SK RNA and the following methylation of the cap were proposed to allow replacement of La with LARP7 [49]. The resulting core 7SK snRNP is composed of the 7SK RNA, MePCE and LARP7 and is boxed in yellow. The C-terminus of LARP7 was shown to inhibit the catalytic activity of MePCE (represented by the red dashed line) and was thus named MePCE inactivating domain (MID). As an alternative to this La-dependent 7SK snRNP maturation pathway, chromatin bound MePCE might directly sequester and methylate the 5ʹ end of nascent 7SK RNA transcripts precluding the access by La and recruiting LARP7 (La-independent pathway, lower left panel)