Nucleolar function of the dense crescent in the yeast nucleus: A biochemical and ultrastructural study

A fraction rich in dense crescent material, as judged by electron microscopy, has been isolated from yeast nuclei. Double-labelling experiments with 14C-uracil and 3H-Me-methionine show that nearly all of the 37S and 28S ribosomal precursor RNA is confined to this dense crescent fraction. The presence of both 37S and 28S ribosomal precursors in the dense crescent fraction strongly suggests that both biosynthesis and processing of the ribosomal precursors take place in the dense crescent. The biochemical and electron microscopic data represent strong evidence that the dense crescent in yeast has to be regarded as a nucleolus in both morphological and biochemical sense. Apparently the yeast cell resembles the higher eukaryotes with regard to the organization within a nucleolus of the synthesis of ribosomal RNA via large precursor molecules.