Fig. 3.

Cotranscriptional processes at the 3′ end of the BR1/BR2 gene. a Schematic representation. Colour code as in Fig. 2. Excision of intron 4 (i4, black box) is initiated rapidly. About 0.6 kb downstream the poly(A) signal (grey box), transcription termination, 3′ end cleavage and polyadenylation are coordinated. The pre-mRNP is retained at the gene during the initial polyadenylation, and splicing is stimulated at this stage. Approximately 3 kb of DNA, as extended chromatin, is present downstream the site of release of the pre-mRNP, before being folded into compact chromatin (black curved line). Released mRNP in dark blue. b EM image of the 3′ end of the gene (adopted from Ericsson et al. 1989). The arrows indicate the chromatin fibre, extending from the final pre-mRNP into the compact chromatin. The scale bar represents 50 nm