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Characterization of an endoribonuclease in rat-liver nucleoli.

Synthetic RNAs, i.e. [3H]poly(U) and [14C]poly(A), and natural RNAs, i.e. 16-S and 23-S Escherichia coli ribosomal [14C]RNA and MS2 coliphage [3H]RNA, were incompletely hydrolyzed by a rat liver nucleolar enzyme as measured by solubilization of radioactivity following ethanol precipitation. Sucrose density gradient analysis of these RNA substrates following incubation with isolated rat liver nucleoli at 36 °C for 15 min demonstrated extensive shifts in their sedimentation coefficients. These data were taken as evidence that the nueleolar enzyme is an endoribonuclease. The enzyme has been solubilized and partially purified. The enzyme has no effect on intact double-stranded reovirus RNA. It cleaves 45-S HeLa precursor rRNA through a series of intermediate fragments of decreasing size to limit 4-S end products. Among other characteristics, this nucleolar endoribonuclease prefers single to multi-stranded RNA and is inhibited by DNAase.