FUNCTIONAL-ANALYSIS OF RRP7P, AN ESSENTIAL YEAST PROTEIN INVOLVED IN PRE-RIBOSOMAL-RNA PROCESSING AND RIBOSOME ASSEMBLY

During the functional analysis of open reading frames (ORFs) identifie d during the sequencing of chromosome III of Saccharomyces cerevisiae, the previously uncharacterized ORF YCL031C (now designated RRP7) was deleted, RRP7 is essential for cell viability, and a conditional null allele was therefore constructed, by placing its expression under the control of a regulated GAL, promoter, Genetic depletion of Rrp7p inhib ited the pre-rRNA processing steps that lead to the production of the 20S pre-rRNA, resulting in reduced synthesis of the 18S rRNA and a red uced ratio of 40S to 60S ribosomal subunits. A screen for multicopy su ppressors of the lethality of the GAL::rrp7 allele isolated the two ge nes encoding a previously unidentified ribosomal protein (r-protein) t hat is highly homologous to the rat r-protein S27. When present in mul tiple copies, either gene can suppress the lethality of an RRP7 deleti on mutation and can partially restore the ribosomal subunit ratio in R rp7p-depleted cells, Deletion of both r-protein genes is lethal; delet ion of either single gene has an effect on pre-rRNA processing similar to that of Rrp7p depletion. We believe that Rrp7p is required for cor rect assembly of rpS27 into the preribosomal particle, with the inhibi tion of pre-rRNA processing appearing as a consequence of this defect.