RNA synthesis in the nucleoli of mouse ascites tumor cells in relation to nucleolar components

Abstract 1. Isolated nucleoli of mouse ascites tumor cells were completely lysed by the action of both sodium polyethylene sulfonate and sodium deoxycholate. The lysate could be separated into the following components by zonal analysis on a sucrose gradient: a rapidly sedimenting, a slowly sedimenting and an intermediate fraction. 2. The sedimentation profile and other properties of the rapidly sedimenting fraction analyzed bore some resemblance to those of cytoplasmic ribosomes. The fraction consisted mainly of ribosomal RNA-containing particles. 3. A kinetic analysis of [2- 14 C]uridine labeling in these components revealed that newly synthesized ribosomal RNA and/or its precursor, which banded slightly ahead of the slowly sedimenting fraction, were progressively transferred to the rapidly sedimenting fraction via the intermediate one. 4. These results thus prove that the nucleoli are a site of the formation of not only ribosomal RNA but also the particles which contain it.