Processing and termination of RNA polymerase I transcripts

Electron micrographs of active ribosomal genes from many species show a similar picture in which gene regions covered with nascent transcripts alternate with apparently non‐transcribed spacers. Since the gradients of visible nascent transcripts stop near the 3′ end of the 28S sequence it has often been assumed that transcription by RNA polymerase I also terminates at that point. Recent biochemical studies have shown however, that transcription continues far beyond the 3′ end of the 28S and in some species continues across the entire spacer. In this article we review the evidence for spacer transcription in Xenopus, mouse and Drosophila and discuss possible reasons for its invisibility in the electron microscope.