We have identified new transcripts in the region surrounding the L-strand replication origin (Ori L) of rat liver mitochondrial DNA. In particular, we have detected previously unidentified intermediates of RNA processing on both the heavy and the light strands, such as precursors of the ND2 mRNA plus the Trp-tRNA and precursors of the tRNAs clustered in the Ori L region. This indicates that the mechanism of RNA processing in mitochondria proceeds step-wise producing a variety of precursors of the mature forms. The other striking finding is the detection of antisense RNA species in the region of L-strand replication. Since a variety of antisense transcripts were also found in the D-loop region of rat mitochondrial DNA, we suggest that they might play a regulatory role in the replication and expression of the mitochondrial genome.
