RPM1 is a mitochondrial gene that encodes an RNA of about 450 nucleotides, also known as the 9S RNA (4, 5). It was originally identified as a locus on the mitochondrial genome that was required for removal of 5' leaders from mitochondrial tRNA precursors (6, 7). The RPM1 RNA comprises one subunit of the enzyme mitochondrial RNase P, which performs 5' processing of pre-tRNAs (2, 8); the other subunit is a protein encoded by the nuclear RPM2 gene (9). Rpm2p is required for the tRNA processing activity of the enzyme, and is also required for maturation of the RPM1 RNA, which does not have the normal 5' or 3' ends in a strain carrying a C-terminally truncated variant of Rpm2p (10).

RPM1 has similarity to the RNA subunits of two-subunit RNases P found in bacteria and protozoa (3, 11). It is conserved in the mitochondrial genomes of other yeasts, but its size varies widely. Some homologs are only about 140 nucleotides in length, suggesting that S. cerevisiae RPM1 may contain nonessential regions (1). As another indication that some regions may be dispensable, the RPM1 RNA becomes fragmented during purification of RNase P from mitochondria, but the enzyme containing RNA fragments remains active (11).

Note:
Three enzyme complexes involved in RNA processing are evolutionarily and physically related, and are easily confused with each other (see 12 and references therein). Mitochondrial RNase P is composed of the mitochondrially-encoded RPM1 RNA and the nuclear-encoded protein Rpm2p; it removes the 5' leaders from mitochondrial tRNA precursors. Nuclear RNase P, which removes the 5' leaders from cytoplasmic tRNA precursors, is composed of the RPR1 RNA subunit and Pop1p, Pop3p, Pop4p, Pop5p, Pop6p, Pop7p, Pop8p, Rpp1p, and Rpr2p. RNase MRP (RNase mitochondrial RNA processing) shares some subunits with nuclear RNase P: it is composed of an RNA subunit encoded by the nuclear NME1 gene and the protein subunits Rpp1p, Snm1p, Pop1p, Pop3p, Pop4p, Pop5p, Pop6p, Pop7p, Pop8p, and Rmp1p. RNase MRP processes pre-rRNAs in the nucleolus and is also present during mitosis in cytoplasmic RNA processing bodies, where it has a role in degradation of daughter cell-specific mRNAs via cleavage of 5' untranslated regions. In mammals, a portion of RNase MRP enters mitochondria and processes RNAs to create RNA primers for DNA replication, but this has not been shown in fungi.

Last updated: 2007-03-01