[Cell-free nuclear assembly induced by nucleoli and demembranated macronuclei from Tetrahymena shanghaiensis]

Nuclei assembled in Xenopus egg extracts from exogenous DNA or chromatin are morphologically similar to normal eukaryotic nuclei except that they do not contain nucleolus-like structure. Previous work show that prenucleolar bodies could be found in these nuclei, but they are unable to fuse with each other, probably due to the absence of a functional nucleolus organizer. It is interesting to know if exogenous functional nuclei could act as functional nucleolus organizer to induce the formation of mature nucleoli. To test this possibility, nucleoli and demembranated macronuclei which contain perinuclear distributed nucleoli were isolated and purified from Tetrahymena shanghaiensis and were introduced into Xenopus cell-free system. By EM observation, we showed that these nucleoli, both individual isolated and perinuclear distributed, could not keep their original morphological characteristic, but undergo a series of structural changes resembling nuclear assembly induced by exogenous chromatin. No nucleolus-like structure was found in these nuclear like structure. These results indicate that functional nucleolus organizers could neither keep their original activity of RNA transcription nor induce the formation of new nucleoli in Xenopus egg extracts.