Mitochondrial 15S rRNA-binding protein%3B required for intron removal of COB and COX1 pre-mRNAs%3B has separable roles in stabilizing mitochondrial 15S rRNA and in maturation of the COB and COX1 mRNAs%3B contains pentatricopeptide repeat (PPR) motifs%3B mutant is respiratory deficient and has defective plasma membrane electron transport