Point mutations distal to the processing site affect Drosophila pre-5 S RNA processing. Long range cooperation and a breathing model

Terminal structure mediates 5 S rRNA stability and integration during ribosome biogenesis

Formation of the eukaryotic ribosomal 5 S RNA-protein complex has been shown to be critical to ribosome biogenesis and has been speculated to contribute to a quality control mechanism that helps ensure that only normal precursors are processed and assembled into active ribosomes. To study the structural basis of these observations, the RNA-protein interface in the 5 S RNA-protein complex of the yeast (Saccharomyces cerevisiae) ribosome was examined based on a systematic introduction of targeted base substitutions in the RNA sequence. Most base substitutions had little or no effect on the efficiency of complex formation, but large effects were observed when changes disrupted helix I, the secondary structure formed between the interacting termini. Again, only modest effects were evident when the extended 3' end of the mature RNA molecule was altered, but essentially no complex was formed when the 5' end of the mature 5 S RNA sequence was artificially extended by one nucleotide. In vitro analyses demonstrated that this extension also dramatically altered the maturation of 5 S rRNA precursor molecules as well as the stability of the mature 5 S rRNA. Taken together, the results indicate that in the course of RNA maturation, the 5 S RNA-binding protein binds precisely over or "caps" the termini in a critical manner that protects the RNA from further degradation.

Ribosomal 5 S rRNA maturation in Saccharomyces cerevisiae

The maturation of the ribosomal 5 S RNA in Saccharomyces cerevisiae is examined based on the expression of mutant 5 S rRNA genes, in vivo, and a parallel analysis of RNA processing, in vitro. Both types of analysis indicate that 5 S rRNA processing is not dependent on the nucleotide sequence of either the external transcribed spacer or the mature 5 S rRNA. The results further indicate the RNA is processed by an exonuclease activity which is limited primarily or entirely by helix I, the secondary structure formed between the mature and interacting termini. The 5 S RNA binding protein (YL3) also appears not to influence directly the maturation process, but rather to play a role in protecting the rRNA from further degradation by "housekeeping" nucleases. Taken together, the results continue to support a "quality control" function which helps to ensure that during maturation only normal precursors are processed and assembled into active ribosomes.

A possible role for the 60-kD Ro autoantigen in a discard pathway for defective 5S rRNA precursors

The Ro autoantigen is a 60-kD protein that is usually found in small cytoplasmic RNA-protein complexes known as Ro RNPs. Although the Ro RNPs are abundant and conserved components of a variety of vertebrate and invertebrate cells, their function is unknown. We have discovered that the Ro protein is also found complexed with certain variant 5S rRNAs in Xenopus oocytes. These RNAs contain one or more point mutations compared with the major oocyte 5S rRNA sequence as well as additional nucleotides at the 3' end. We demonstrate that the Ro protein binds specifically mutant 5S rRNAs containing 3' terminal extensions. These mutant RNAs are processed inefficiently to mature 5S rRNA and most eventually are degraded. The observation that the Ro autoantigen specifically associates with defective 5S rRNA precursors suggests that this protein may function as part of a novel quality control or discard pathway for 5S rRNA production.