Isolation of a 5S RNA-protein L5 complex from 60S subunits of rat liver ribosomes by cesium sulfate density-gradient equilibrium centrifugation.

Upon CS2SO4 density-gradient equilibrium centrifugation, 60S subunits of rat liver ribosomes were dissociated to form three bands at the densities of 1.55, 1.40, and 1.30 g/ml. The bands at 1.55 and 1.30 g/ml were shown to contain exclusively RNA or proteins, respectively, whereas the band at 1.40 g/ml contained both RNA and a protein. The RNA in the 1.40 g/ml band was identified as 5S RNA and the protein in this band was protein L5. An empirical calculation suggested that the 1.40 g/ml band contained an equimolar 5S RNA-protein L5 ribonucleoprotein complex. This finding may indicate that protein L5 is located in very close proximity to 5S RNA in large ribosomal subunits.