The exosome subunit Rrp43p is required for the efficient maturation of 5.8S, 18S and 25S rRNA

The Saccharomyces cerevisiae protein Rrp43p copurifies with four other 3'-- >5' exoribonucleases in a complex that has been termed the exosome, Rrp43p itself is similar to prokaryotic RNase PH, Individual exosome subunits have been implicated in the 3' maturation of the 5.8S rRNA found in 60S ribosom es and the 3' degradation of mRNAs, However, instead of being deficient in 60S ribosomes, Rrp43p-depleted cells were deficient in 40S ribosomes, Pulse -chase and steady-state northern analyses of pre-RNA and rRNA levels reveal ed a significant delay in the synthesis of both 25S and 18S rRNAs, accompan ied by the stable accumulation of 35S and 27S pre-rRNAs and the under-accum ulation of 20S pre-rRNA, In addition, Rrp43p-depleted cells accumulated a 2 3S aberrant pre-rRNA and a fragment excised from the 5' ETS. Therefore, in addition to the maturation of 5.8S rRNA, Rrp43p is required for the maturat ion 18S and 25S rRNA.