The intervening sequence in the 26S rRNA coding region of T. thermophila is transcribed within the largest stable precursor for rRNA.

We studied the transcription of the intervening sequence in the 26S rRNA coding region of the extrachromosomal rDNA molecules in the macronucleus of T. thermophila by hybridization of purified nuclear rRNA precursors or cytoplasmic 26S rRNA to purified native rDNA or specific rDNA restriction fragments. Examination of R loop hybrids in the electron microscope and analyses of S1-protected rDNA fragments in alkaline agarose gels showed that mature 26S rRNA, nuclear pre-26S rRNA and a fraction of the pre-rRNA molecules containing both the sequences for 17S and 26S rRNA all lack the region corresponding to the intervening sequence. The rest of the pre-rRNA molecules, however, hybridize in a colinear fashion to the whole coding region, and thus must contain the intervening sequence. We can conclude from these results that the intervening sequence is transcribed within the primary transcription product of the rDNA, and that the post-transcriptional removal of the intervening RNA sequence is a very early processing event in the organism.