Bypassing the rRNA processing endonucleolytic cleavage at site A2 in Saccharomyces cerevisiae.

Rrp5p is the only ribosomal RNA processing trans-acting factor that is required for the synthesis of both 18S and 5.8S rRNAs in Saccharomyces cerevisiae. Mutational analyses have characterized modified forms of Rrp5p that either affect formation of 18S rRNA by inhibiting cleavage at sites A0/A1/A2, or synthesis of 5.8S rRNA by inhibiting cleavage at site A3. Here, we examine the rRNA maturation process associated with a RRP5 bipartite allele that codes for two noncontiguous parts of the protein. This slow-growing bipartite mutant has a unique rRNA-processing phenotype that proceeds without endonucleolytic cleavage at site A2. In wild-type cells, the A2 cleavage takes place on the 32S pre-rRNA and is responsible for the formation of 20S and 27SA2 species, the precursors of mature 18S and 5.8S/25S rRNAs, respectively. In the bipartite strain, such precursors were not detectable as judged by Northern analysis or in vivo labeling. They were replaced by the aberrant 21S species and the bypassing 27SA3 precursor, both descended from direct cleavage of 32S pre-rRNA at site A3, which provides an alternative rRNA maturation pathway in this strain. The 21S pre-rRNA is the sole detectable and most likely available precursor of 18S rRNA in this particular strain, indicating that 18S rRNA can be directly produced from 21S. Furthermore, 21S species were found associated with 43S preribosomal particles as similarly observed for the 20S pre-rRNA in the wild-type cells.