Nucleolar transcription during plant mitosis. In situ assay for RNA polymerase activity

Allium cepa L. root meristems, after a brief fixing in cold methanol, retain endogenous RNA polymerase activity. This is shown by autoradiography of meristems squashed after being incubated with the four ribonucleotides, UTP being 3H-labelled. The pattern of labelling after this assay is similar to that shown after in vivo incorporation of [3H]cytidine. Nucleolar transcription appears to be as important in prophase as in interphase and under our conditions no mitotic cells present significant RNA polymerase activity outside the nucleolus. The nucleolar transcription is timed in relation to chromosome and nucleolar cycles. It stops at the very end of prophase, shortly before the last nucleolar remnants disappear. Reinitiation of RNA polymerase activity is found in organizer regions of nucleolar chromosomes, where it first appears in mid-telophase. This work also supplies a new evidence in favour that prenucleolar bodies are not synthesized in telophase since they appear before any transcription reinitiates