EM visualization of transcription by RNA polymerase II: Downstream termination requires a poly(A) signal but not transcript cleavage

We have used EM visualization of active genes on plasmid vectors in Xenopus oocyte nuclei to investigate the relationship between poly(A) signals and RNA polymerase II transcription termination. Although a functional poly(A) signal is required for efficient termination, cotranscriptional RNA cleavag e at the poly(A) site is not. Furthermore, the phenomena of termination and cotranscriptional RNA cleavage can be uncoupled, and the efficiency of bot h varies independently on different copies of the same plasmid template in the same oocyte nucleus. The combined observations are consistent with a sc enario in which there is template-specific addition to Pol II (presumably a t the promoter) of elongation and/or RNA processing factors, which are alte red upon passage through a poly(A) signal, resulting in termination and, in some cases, cotranscriptional RNA cleavage.