Chromatin structure of Xenopus rDNA transcription termination sites. Evidence for a two-step process of transcription termination.

The ultrastructure of Xenopus laevis (X.l.) 40S pre-rRNA transcription termination sites was investigated by electron microscopy. Amplified nucleolar chromatin was rapidly dispersed and processed for chromatin spread preparations. This type of preparation revealed that many rDNA termination sites appeared as 50 nm long segments of chromatin axis covered by a complex of three closely spaced RNA polymerase particles. Particle (Ptl) was characterized by the association with the terminal full-length pre-rRNP fibril; particles (Pt2) and (Pt3) are located downstream from (Ptl) and appear to be devoid of transcript fibrils. This particular structural arrangement as well as sequence homology analyses of 3' adjacent spacer rDNA segments indicate that transcript release and dissociation of polymerase particles are not necessarily coupled and termination of X.l. rDNA transcription may occur as a two step process. The structural data are correlated with homologies of DNA sequences at Xenopus rDNA transcription termination regions and are discussed with respect to sequence data of 3' termination sites of rRNA genes from other species.