Subnucleolar location of fibrillarin and variation in its levels during the cell cycle and during differentiation of plant cells.

The nucleolar protein fibrillarin has been studied in onion cells; it is detected as an M(r) 37,000 protein by immunoblotting using a human autoimmune serum. Quantitative immunoelectron microscopy showed that most fibrillarin is localized in the transition zone between the fibrillar center (FC) and the dense fibrillar component (DFC) as well as in the proximal zone of the DFC, where the labeling shows a gradual decrease outward until it reaches insignificant levels in the distal zone of the DFC. Thus, fibrillarin is not uniformly distributed throughout the DFC of plant cells. This result supports the hypothesis that the morphologically homogeneous DFC may not be uniform in function; it is also in agreement with the hypothesized vectorial flow of ribosome biogenesis through the same compartments. Data are also presented showing that the amount of fibrillarin increases when nucleolar activity increases in G2, and probably decreases when nucleolar activity decreases during differentiation.