Guide RNA molecules not engaged in RNA editing form ribonucleoprotein complexes free of mRNA.

Mitochondrial pre-mRNAs in kinetoplastid organisms undergo uridine additions and deletions after transcription, a phenomenon termed kRNA editing. The reaction involves small, mitochondrial DNA transcripts, so called guide RNAs which provide the editing information via base pairing to the pre-mRNAs and furthermore may act as the U-nucleotide donors. Guide RNAs are not maintained as free molecules within the mitochondrial organelle, instead form several high molecular weight ribonucleoprotein complexes. Here we report the identification of two new gRNA containing RNP complexes, 8S and 15S in size, that only assemble with upstream gRNA molecules which require editing of their cognate pre-mRNA before they can base pair. The two complexes do not contain pre-mRNA molecules and the 8S RNP can be assembled in vitro. It contains two polypeptides under these conditions with apparent molecular weights of 90 and 21 kDa that can be cross-linked to the gRNA molecule. Our observation suggests the existence of structurally simple gRNA/protein complexes that might function as building blocks for the assembly of a high molecular weight editing machinery.