Mapping and characterization of Paracentrotus lividus mitochondrial transcripts: multiple and overlapping transcription units.  This paper reports the mapping of both mature and precursor Paracentrotus lividus mitochondrial transcripts. Several mtRNAs were found to have 5'' and 3'' termini which differ from those inferred through DNA sequencing (Cantatore et al. 1989). The 3'' ends of the two rRNAs (12S and 16S) overlap with the downstream transcripts (tRNAGlu and CoI mRNA) by 5 and 10 nt respectively. The 132 nt non-coding region is extensively transcribed: in particular it contains a 124 nt RNA and the 5'' end of a possible precursor of 13 clustered tRNAs. This latter overlaps by 7 nt with the 3'' end of the 124 nt RNA. In addition to the mature RNAs, 32 high molecular weight RNAs, which are probably the precursors of the smaller more abundant mature species, were detected by Northern blotting. The mapping of these transcripts indicates that they are processed at the level of tRNA or tRNA-like sequences and suggests the existence of two transcription initiation sites upstream of the ND1 and the cytochrome b genes respectively. In the light of these results it appears that P. lividus mitochondrial DNA transcription takes place via multiple and probably overlapping transcription units. Moreover, the wide variation in the steady-state levels of the mature mRNAs indicates that sea urchin mitochondrial DNA expression is also regulated at the level of RNA decay.