The mechanism of mitochondrial transcription is well documented although the method of regulation remains obscure. The mitochondrial transcription-termination complex, mTERF, holds a key position in determining the fate of heavy-strand promotor-initiated transcripts and has been suggested as a candidate in the regulation of mitochondrial DNA (mtDNA) transcription. We report here the first example of a modulation of mTERF-complex binding activity concomitant with a differential mtDNA transcription rate. We suggest that these observations are indicative of a method of intra-organellar transcriptional fine tuning.