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 cleavage at the poly(A) site is not. Furthermore, the phenomena of termination and cotranscriptional RNA cleavage can be uncoupled, and the efficiency of both varies independently on different copies of the same plasmid template in the same oocyte nucleus. The combined observations are consistent with a scenario in which there is template-specific addition to Pol II (presumably at the promoter) of elongation and/or RNA processing factors, which are altered upon passage through a poly(A) signal, resulting in termination and, in some cases, cotranscriptional RNA cleavage.
Osheim
Y N
YN
University of Virginia Health Sciences Center, Department of Microbiology, Charlottesville 22908, USA.
Proudfoot
N J
NJ
Beyer
A L
AL
D000818:Animals; D004268:DNA-Binding Proteins / Q000235:genetics; D005796:Genes / Q000235:genetics; D008854:Microscopy, Electron; D009865:Oocytes / Q000378:metabolism; D010957:Plasmids / Q000235:genetics / Q000648:ultrastructure; D011061:Poly A / Q000235:genetics*; D011401:Promoter Regions, Genetic / Q000235:genetics; D012319:RNA Polymerase II / Q000378:metabolism*; D012333:RNA, Messenger / Q000378:metabolism* / Q000648:ultrastructure; D020218:Response Elements / Q000235:genetics; D013698:Templates, Genetic; D035582:Transcription Factor TFIIIA; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism; D014158:Transcription, Genetic / Q000235:genetics*; D014982:Xenopus laevis
D004268:DNA-Binding Proteins; D012333:RNA, Messenger; D035582:Transcription Factor TFIIIA; D014157:Transcription Factors; D011061:Poly A; D012319:RNA Polymerase II