Evidence for specific association of protein with newly formed ribosomal subunits

Abstract

Experiments on the kinetics of labeling of Dictyostelium discoideum ribosomes with [3H]uridine have demonstrated the existence of the precursor ribonucleoprotein particles to the large and small ribosomal subunits. The precursor particle to the large (50 S) subunit sediments at approx. 43 S and 47 S in high ionic strength sucrose gradients and that to the small (30 S) subunit at 25 S.The 43-S and 47-S precursor particles contain mainly 28-S preribosomal RNA and their buoyant density in CsCl is ρ = 1.595, characteristic of mature 50-S subunit. The 25-S precursor particle contains 17-S rRNA and its buoyant density is probably higher than the density (ρ = 1.530) of mature 30-S subunit.Examination of RNAs labeled with [Me-3H]methionine has shown that 36-S, 30-S and 19-S preribosomal RNAs of this organism also exist as ribonucleoprotein complexes sedimenting in the region from approx. 55 S to 100 S.The assembly of the ribosomal subunits was arrested upon the inhibition of protein synthesis by cycloheximide, but the 47-S and 25-S ribonucleoprotein particles accumulated. These particles were transferred to mature subunits upon removal of the drug from the culture. The 47-S particle was converted also in vitro to the 50-S subunit in presence of a large quantity of the 105 000 × g supernatant fluid of the crude extract, but this was not the case for the 25-S particle, precursor to the 30-S subunit.It has been suggested that protein synthesis participates in the conversion of the 47-S and 25-S precursor particles to mature ribosomal subunits.