A restriction fragment length polymorphism in the 5' non-transcribed spacer of the rDNA of Tetrahymena thermophila inbred strains B and C3.

The ribosomal DNA of Tetrahymena thermophila is a 21-kb palindromic molecule which replicates autonomously in the macronuclei of this species. In addition to the rRNA coding regions, there are 5' and 3' flanking sequences which are not transcribed (non-transcribed spacer; NTS). The 5' NTS contains a bidirectional origin of DNA replication and promoter elements which direct transcription. We have identified a restriction fragment length polymorphism in the rDNA 5' NTS by comparing the B and C3 inbred strains of T. thermophila. There is a 42-bp region present in the C3 but not in the B strain rDNA; we present evidence that this difference most likely represents a deletion in the B strain rather than an insertion in the C3 strain. We also include a revised version of the nucleotide sequence of the 5' NTS DNA of inbred strain B.