Unusual transcription termination of the ribosomal RNA genes in Ascaris lumbricoides.

We studied termination of transcription of the ribosomal RNA genes in Ascaris lumbricoides, the first representative in the phylum of nemathelminthes analysed so far. RNase protection experiments in vivo reveal that the 3' end of the precursor rRNA coincides with the end of mature 26S rRNA. Promoter-containing miniplasmids are able to direct unique 3' end formation in vitro at a site identical to that observed in vivo, whereas deletion of these sequences abolishes 3' end formation throughout the entire spacer. A nuclear run-on experiment in vitro confirms the drop of polymerase I concentration down-stream of this site. The termination site for polymerase I transcription of the rDNA operon in A. lumbricoides is therefore unique, and located at the very end of the 26S rRNA gene.