Abstract
RNA of isolated nucleoli of rat liver was subjected to sucrose density-gradient centrifugation. The major components were 45-S, 35-S and 28-S RNA, although a smaller peak of 6-S RNA and several other minor components were also present. Extranucleolar nuclear RNA contained mainly 28-S, 18-S and 6-S RNA. With radioactive orotic acid as a precursor and with the aid of actinomycin D as a blocking agent of new RNA synthesis, evidence was obtained that the 45-S RNA is biosynthesized in the nucleolus and converted there into 35-S RNA and 28-S RNA. It was found that the nucleolar RNA contains little 18-S RNA. The base composition of whole nucleolar RNA supports the evidence that the nucleolus synthesizes precursors of the 50–60-S subunits of the ribosomes. Evidence for the rapid turnover of the 45-S RNA in nucleoli was provided by studies on the kinetics of the decay of the 45-S peak in actinomycin D-treated nucleoli which showed that the half-life of the 45 S was approx. 8 min.
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