In S. cerevisiae, the large oxi3/oli2 mitochondrial transcript contains the products of the oxi3, aap1 and oli2 genes and an unassigned reading frame, RF3. In the work presented here, we have completed the nucleotide sequence of RF3. We have shown that RF3 is composed of four fairly large ORFs which overlap within GC rich sequences. Furthermore, a shift of +1 base was found between each pair of consecutive reading frames. We discuss how these frameshifts could be removed to produce a 500 aminoacid long protein containing the two well conserved P1 and P2 oligopeptide sequences featuring several mitochondrial intron reading frames, suggesting, thereby, a RNA-maturase-like activity for the putative RF3 protein. In addition, we suggest that the insertion of GC clusters in a gene could provide a novel way of regulating its expression.