Localization of transcribed regions on extrachromosomal ribosomal RNA genes of Tetrahymena thermophila by R-loop mapping.

R-loop hybridization and electron microscopy were used to map the RNA transcription products of the extrachromosomal rRNA genes of Tetrahymena thermophila. The mature 17S and 26S rRNAs and the nuclear 35S pre-rRNA and pre-26S rRNA were located with a precision of approximately 100 base pairs. A 370-base pair intervening sequence was found in the 26S coding region. It has the same size and relative location as that found in Tetrahymena pigmentosa [Wild, M.A. & Gall, J.G. (1979) Cell 16, 565-573]. One class of R-loop structures formed by nuclear pre-rRNA provided preliminary evidence for a primary transcript that contains the intervening sequence. The results suggested a processing scheme in which splicing of the intervening sequences is followed by a series of strand cleavages to give the mature 17S and 26S rRNAs. Analysis of the data also showed that RNA . DNA and DNA . DNA duplexes, when mounted for electron microscopy by the R-loop procedure, have the same length per base pair within 4%.