| Summary: | Ccm1p is a nuclear-encoded PPR (pentatricopeptide repeat) protein that localizes into
mitochondria of Saccharomyces cerevisiae. It was first defined as an essential
factor to remove the bI4 [COB (cytochrome b) fourth intron)] and
aI4 [COX1 (cytochrome c oxidase subunit 1) fourth intron] of
pre-mRNAs, along with bI4 maturase, a protein encoded by part of bI4 and preceding exons that
removes the intronic RNA sequence that codes for it. Later on, Ccm1p was described as key to
maintain the steady-state levels of the mitoribosome small subunit RNA (15S rRNA). bI4 maturase is
produced inside the mitochondria and therefore its activity depends on the functionality of
mitochondrial translation. This report addresses the dilemma of whether Ccm1p supports bI4 maturase
activity by keeping steady-state levels of 15S rRNA or separately and directly supports bI4 maturase
activity per se. Experiments involving loss of Ccm1p, SMDC (sudden mitochondrial
deprivation of Ccm1p) and mutations in one of the PPR (pentatricopeptide repeat) motifs revealed
that the failure of bI4 maturase activity in CCM1 deletion mutants was not due to a
malfunction of the translational machinery. Both functions were found to be independent, defining
Ccm1p as a moonlighting protein. bI4 maturase activity was significantly more dependent on Ccm1p
levels than the maintenance of 15S rRNA. The novel strategy of SMDC described here allowed the study
of immediate short-term effects, before the mutant phenotype was definitively established. This
approach can be also applied for further studies on 15S rRNA stability and mitoribosome
assembly.
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