3'-Inverted repeats in plant mitochondrial mRNAs are processing signals rather than transcription terminators

A number of mRNAs in plant mitochondria contain inverted repeats at their 3'-termini. These have been discussed as potential transcription terminators or, alternatively, as post-transcriptional processing and stability signals of longer precursor RNAs. In vitro transcription in a pea mitochondrial lysate now shows that transcription proceeds almost unimpeded through these inverted repeat structures. To investigate their potential function in mRNA processing, we developed an in vitro processing system from pea mitochondria. This in vitro system correctly processes synthetic precursor mRNAs containing the pea atp9 double stem-loop structure, yielding the same 3'-termini observed in vivo. Analysis of the in vitro-generated products and of the processivity of the reaction suggests exonucleolytic degradation up to the stem-loop. The inverted repeat structures found at the 3'-termini of mRNAs in plant mitochondria are thus recognized as processing and most likely also stabilizing signals in transcript maturation, but do not terminate transcription.