Splicing of the group I intron aI5 beta from the yeast mitochondrial COX1 transcript requires at least four proteins, encoded by the nuclear genes PET54, MRS1/PET157, SUV3 and MSS18. These proteins either act directly to facilitate intron aI5 beta excision, or indirectly in some manner. One possible indirect mode of action of these nuclear gene products is in stimulation of expression of a mitochondrial protein, such as a maturase, that is necessary for intron aI5 beta excision. To test this possibility, splicing of intron aI5 beta was examined in a rho-strain, which is incapable of mitochondrial protein synthesis. A quantitative RT-PCR assay was set up to compare levels of spliced COX1 mRNA present in three strains: a wild-type rho + strain; the rho-strain 7-49b-11, which retains the entire COX1 transcription unit; and a strain bearing a null mutation in the nuclear PET54 gene. The results showed that excision of aI5 beta occurs relatively efficiently in the rho-strain, and therefore does not require any mitochondrial-encoded proteins.