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Abstract
tRNAs are synthesized as immature precursors, and on their way to functional maturity,
extra nucleotides at their 5' ends are removed by an endonuclease called RNase P.
All RNase P enzymes characterized so far are composed of an RNA plus one or more proteins,
and tRNA 5' end maturation is considered a universal ribozyme-catalyzed process. Using
a combinatorial purification/proteomics approach, we identified the components of
human mitochondrial RNase P and reconstituted the enzymatic activity from three recombinant
proteins. We thereby demonstrate that human mitochondrial RNase P is a protein enzyme
that does not require a trans-acting RNA component for catalysis. Moreover, the mitochondrial
enzyme turns out to be an unexpected type of patchwork enzyme, composed of a tRNA
methyltransferase, a short-chain dehydrogenase/reductase-family member, and a protein
of hitherto unknown functional and evolutionary origin, possibly representing the
enzyme's metallonuclease moiety. Apparently, animal mitochondria lost the seemingly
ubiquitous RNA world remnant after reinventing RNase P from preexisting components.