Processing of the 3' sequence extensions upon the 5S rRNA of a mutant yeast in Xenopus laevis germinal vesicle extract.

A processing endonuclease acts to remove a short sequence from the 3' end of transcripts of the Saccharomyces cerevisiae 5S ribosomal RNA gene in generating the mature sequence of 5S RNA. Cells bearing the nuclear mutation rna82 .1 lack this activity and accumulate 5S forms with additional nucleotides at their 3' termini. 5S RNAs labelled during short pulse- labellings of the mutant are essentially primary transcripts that mostly have the sequence U-U-A-U-U-U-C[U-U-U-U(U-U)] added to the 3' end of normal yeast 5S RNA. They are subjected in vivo to a series of slow processing events whereby this sequence is ultimately replaced by: U-U(A)1-9 in a substantial proportion of the 5S RNA molecules of the mutant [Piper, P. W., Bellatin , J. A. and Lockheart , A. (1983) EMBO J. 2, 353-359]. In higher eukaryotes no endonuclease cleavage occurs during 5S RNA maturation, yet processing at the 3' ends of certain transcripts made by RNA polymerase III, most notably transfer RNA precursors, is still important. Since the enzymes involved in this processing have not been well characterised, we investigated how the additional sequences upon rna82 .1 yeast 5S RNA are processed in vitro in a system from a higher eukaryote that is often used for studying transcription by RNA polymerase III, the Xenopus laevis germinal vesicle extract. Our results are consistent with slow digestion of these 5S molecules by a 3'----5' exonuclease until they become 122-123 nucleotides in length, whereupon digestion ceases. This activity probably participates in the processing of certain Xenopus RNA polymerase III transcripts.