EXPERIMENTAL ANALYSIS OF NUCLEOLAR REORGANIZATION

Abstract

Biochemical studies carried out on cells in transition from telophase to G1 have shown that protein and RNA syntheses, which are only slight in the middle stages of mitosis, are rapidly intensified towards the end of telophase, precisely during the period of nucleolar reorganization. In line with this finding, it is thought that mitosis plays a role in realigning the transcriptional pattern of the cell cycle. It was therefore considered that it would be of interest to study nucleolar reorganization under inhibition of either protein or RNA synthesis in order to determine the processes on which nucleologenesis depends. This was accomplished in Allium, where the whole set of ribosomal cistrons are clustered in the nucleolar organizer region (NOR) of one pair of homologous chromosomes, and these give rise to just one pair of nucleoli in the interphasic nuclei. It appeared that nucleolar reorganization is a process fully dependent on simultaneous RNA synthesis. The possibility that a non nucleolar RNA is produced that is specific for the activation of transcription of ribosomal cistrons was investigated by means of α amanitin. This seems not to be the case. This inhibitor slows down the initiation of nucleolar reorganization in the first 2 hr of treatment and then speeds up the process subsequently so the normal rate of nucleolar reorganization is attained after the 4th hr. With respect to the role of protein synthesis in nucleologenesis, there was an increase in the rate of nucleolar reorganization compared with the normal course of the process. Prenucleolar bodies seem to group themselves together to form fully developed nucleoli at almost double the rate at which they do so in untreated cells.