The pattern of transcription of the human mitochondrial rRNA genes reveals two overlapping transcription units.

A detailed analysis of the mapping and kinetic properties of oligo(dT)-cellulose bound and unbound transcripts synthesized in HeLa cells has indicated that two distinct transcription events take place in the rDNA region. Of these, one appears to start approximately 25 bp upstream of the tRNAPhe gene and to terminate at or near the 3' end of the 16S rRNA gene, being responsible for the synthesis of the bulk of rRNA. The other transcription event starts near the 5' end of the 12S rRNA gene, proceeds beyond the 3' end of the 16S rRNA gene, and results in the synthesis of a polycistronic molecule corresponding to almost the entire H-strand, which is destined to be processed to yield the mRNAs and most of the tRNAs encoded in the heavy strand. The existence of two overlapping transcription units with distinct promoters is probably the basis for the differential regulation of synthesis of the rRNAs and heavy-strand-coded mRNAs.