5-S RNA: biosynthesis and association to immature ribosomal particles in the nucleus of lymphocytic mouse leukemia cells, L5178 Y.

Abstract

In the nucleus of L5178 Y cells, a lymphocytic mouse leukemia cell strain, newly synthesized 5‐S ribosomal RNA first appears mainly in the nucleoplasm and then diffuses into the nucleolus. Kinetic studies have revealed that the appearance rates of 5‐S RNA and “45‐S fraction” ribosomal precursor RNA are identical. During the first 30 min, 5‐S RNA binds to proteins and constitutes a nuclear pool of independent ribonucleoprotein particles which sediment at approximately 11 S. After this time, it begins to integrate with the nucleolar “80‐S nascent” particle. This ribonucleoprotein particle contains “45‐S fraction” RNA, the direct product of ribosomal DNA genes and has been shown to represent the first stage of ribosome formation. 5‐S RNA remains associated to preribosomal particles during the nuclear maturation process leading to the large ribosomal subunit, located in the cytoplasm. In labeling experiments, the molar ratio of labeled 5‐S RNA compared to high‐molecular‐weight ribosomal RNA in ribonucleoprotein particles, continues to increase until reaching a value of 1:1 for steady‐state labeling. The association of 5‐S RNA to preribosomal particles is more labile than it is to the mature ribosomal subunit. Chemical products such as heparin or sarkosyl used to extract nucleolar ribonucleoprotein particles, were shown to detach 5‐S RNA from these particles; but dithiothreitol keeps the particles intact.