SRD1, a S. cerevisiae gene affecting pre-rRNA processing contains a C2/C2 zinc finger motif

The Saccharomyces cerevisiae genes, RRP1 and SRD1, are involved in processing rRNA precursor species to mature rRNAs. We reported previously that the rrp1-1 mutation caused temperature-sensitive lethality, hypersensitivity to aminoglycoside antibiotics, and defective processing of 27S pre-rRNA to 25S and 5.8S mature rRNAs. A second-site suppressor of the rrp1-1 mutation, srd1, corrects all three rrp1 mutant phenotypes. In order to learn more about the roles of the SRD1 and RRP1 genes in rRNA processing, we cloned and characterized the SRD1 gene. We identified an ORF, YCR18C, that complements srd1-2 suppression of rrp1-1. The DNA is physically located at the region of chromosome III where SRD1 has been genetically mapped. SRD1 encodes a putative 225 amino acid, 26 kDa protein containing a C2/C2 zinc finger motif that is also found in some transcription regulators and the eIF-2 beta translation initiating factors. The similarity of SRD1 to transcription regulators led us to test the model that srd1 mutations suppress rrp1 defects by altering the level of the RRP1 transcript. However, we found that SRD1 has no detectable effect on the steady state levels of RRP1 mRNA. We describe alternative models to explain the role of Srd1p in pre-rRNA processing.