Model of cotranscriptional regulation of 7SL and BC200 RNA by SRP9/SRP14. Schematic depicting genetic loci of 7SL and BC200 (rectangle), DNA (double helix), RNA polymerase III and associated transcription machinery (pentagon), SRP9/SRP14 (oval), and 7SL and BC200 RNA (outlines of the RNA from those observed in Fig. 1). Proteins, RNA, DNA, and genes are not to scale. SRP9/SRP14 are distributed nonspecifically across the DNA at high abundance. (i) SRP9/SRP14 promote the initiation of RNA polymerase III and associated transcription machinery to the indicated genetic locus; (ii) SRP9/SRP14 associated with DNA at close proximity to the specified genetic locus interact with the evolutionarily conserved structure formed by the first ∼50 nt of 7SL and BC200 RNA while transcription is ongoing (Strub et al. 1991; Emde et al. 1997; Weichenrieder et al. 1997); and (iii) full-length nascent 7SL and BC200 RNA interacting with SRP9/SRP14 are synthesized, while those lacking SRP9/SRP14 are degraded and transcription is aborted. This model is consistent with the proposed functions of SRP9/SRP14 in Alu retrotransposition (Dewannieux et al. 2003; Bennett et al. 2008). Nascent 7SL and BC200 RNA are then capable of continuing their normal life cycle, which could include export (He et al. 1994; Grosshans et al. 2001; Sommerville et al. 2005), posttranscriptional processing (Chen et al. 1998; Perumal et al. 2001; Massenet 2019), among other functions.
