Proteasome mutants, pre4-2 and ump1-2, suppress the essential function butnot the mitochondrial RNase P function of the Saccharomyces cerevisiae gene RPM2

The Saccharomyces cervisiae nuclear gene RPM2 encodes a component of the mi tochondrial tRNA-processing enzyme RNase P. Cells grown on fermentable carb on sources do not require mitochondrial tRNA processing activity, but still require RPM2, indicating an additional function for the Rpm2 protein. RPM2 -null cells arrest after 25 generations on fermentable media. Spontaneous m utations that suppress arrest occur with a frequency of similar to 9 X 10(- 6). The resultant mutants do not grow on nonfermentable carbon sources. We identified two loci responsible for this suppression, which encode proteins that influence proteasome function or assembly. PRE4 is an essential gene encoding the beta-7 subunit of the 20S proteasome core. X Val-to-Phe substi tution within a highly conserved region of Pre4p that disrupts proteasome f unction suppresses the growth arrest of RPM2-null cells on fermentable medi a. The other locus, UMP1, encodes a chaperone involved in 20S proteasome as sembly. A nonsense mutation in UMP1 also disrupts proteasome function and s uppresses Delta rpm2 growth arrest. In an RMP2 wild-type background, pre4-2 and ump1-2 strains fail to grow at restrictive temperatures on nonfermenta ble carbon sources. These data link proteasome activity with Rpm2p and mito chondrial function.