Structural changes in nucleoli during inhibition of protein- and RNA-biosynthesis.

A variety of cells (unstimulated human lymphocytes, phytohemagglutinin-stimulated human lymphocytes, diploid human fibroblasts, human melanoma cells, and Hela cells) were subjected in vitro to inhibition of protein biosynthesis by puromycin. Hela cells were also treated with actinomycin D to inhibit RNA-synthesis. Under puromycin treatment, the fibrillar centers of the nucleoli were smaller in all actively dividing cell types, whereas in small inactive lymphocytes from peripheral blood the inhibition of protein synthesis had no noticeable effect. Nucleoli with nucleolonema changed into compact nucleoli under puromycin treatment. When RNA-synthesis was inhibited, the fibrillar centers remained at an approximately constant volume. These findings indicate that proteins localized in the fibrillar centers are involved in, and are used up during, rDNA-transcription and/or further steps of ribosome biogenesis. The changes in nucleolar architecture after the inhibition of protein synthesis suggest that transcriptional processes become concentrated near sites where proteins have been stored, i.e. the fibrillar centers.