The production of ribosomal RNA from high molecular weight precursors: III. Hydrolysis of pre-ribosomal and ribosomal RNA by a 3′-OH specific exoribonuclease

Abstract

Highly purified preparations of a nuclear exoribonuclease were used to hydrolyze individual ribosomal and pre-ribosomal RNA components. The kinetics of hydrolysis of 45, 32, 28 and 18 s ribosomal RNA were studied and the frequency of methylated nucleotides near the 3′-OH termini of the various rRNA components was determined. The results indicate that there are methyl-deficient segments greater than 600 nucleotides long at the 3′-OH ends of both the 45 s and 32 s molecules, and that these segments can be hydrolyzed by the exonuclease at rates which are at least an order of magnitude greater than those observed for the mature 28 s and 18 s species. There is a great deceleration in rate, however, when the enzyme reaches certain internal points within the precursor molecules. The foregoing data are discussed in regard to the possible arrangements of the 28 s and 18 s segments within the 45 s molecule and to the possible enzymic mechanisms for rRNA processing.