GUIDE RNA MOLECULES NOT ENGAGED IN RNA EDITING FORM RIBONUCLEOPROTEINCOMPLEXES FREE OF MESSENGER-RNA

Mitochondrial pre-mRNAs in kinetoplastid organisms undergo uridine add itions and deletions after transcription, a phenomenon termed kRNA edi ting, The reaction involves small, mitochondrial DNA transcripts, so c alled guide RNAs which provide the editing information via base pairin g to the pre-mRNAs and furthermore may act as the U-nucleotide donors. Guide RNAs are not maintained as free molecules within the mitochondr ial organelle, instead form several high molecular weight ribonucleopr otein complexes. Here we report the identification of two new gRNA con taining RNP complexes, 85 and 15S in size, that only assemble with ups tream gRNA molecules which require editing of their cognate pre-mRNA b efore they can base pair. The two complexes do not contain pre-mRNA mo lecules and the 8S RNP can be assembled in vitro. It contains two poly peptides under these conditions with apparent molecular weights of 90 and 21 kDa that can be cross-linked to the gRNA molecule. Our observat ion suggests the existence of structurally simple gRNA/protein complex es that might function as building blocks for the assembly of a high m olecular weight editing machinery.