Structural features in the 3' external transcribed spacer affecting intragenic processing of yeast rRNA.

A highly conserved extended hairpin structure in the 3' external transcribed spacer (3' ETS) region of nascent eukaryotic rRNA transcripts is essential for the maturation of the large ribosomal subunit RNAs (5.8 S and 25 to 28 S rRNAs). Systematic changes were introduced into this structure by PCR-mediated mutagenesis and the mutant rDNAs were expressed in vivo to determine the structural features that are essential for rRNA maturation. Changes in the lower half of the stem or the large loop at the end had little or no effect on the maturation of either the 5.8 S or 25 S rRNA, but changes that disrupted secondary structure in the upper half of this stem had equal and dramatic effects on both RNAs. When the RNA stem was incubated with a cellular protein extract, gel retardation studies indicated that the stem forms a ribonucleoprotein complex, and a comparison with mutant RNA indicated that protein binding could be compromised by changes that were critical for rRNA maturation. Sequence comparisons with other spacer regions as well as snRNAs reveal some structural analogy, which, when taken together with the mutational studies, raise the possibility that this hairpin functions during RNA processing in a manner that may be analogous with that of free snRNPs.