Molecular evidence for somatic recombination in the ribosomal DNA of Tetrahymena thermophila.

The ribosomal DNA (rDNA) in Tetrahymena thermophila is a 21-kilobase-pair palindromic DNA molecule that replicates autonomously in the macronucleus and is maintained at the level of about 10,000 copies per macronucleus. The rDNA of inbred strain C3 outreplicates the rDNA of inbred strain B in most B/C3 heterozygous macronuclei, generating macronuclei containing exclusively C3 rDNA sequences. In 1% or less of the B/C3 heterozygous macronuclei, however, rDNA sequences derived from both B and C3 strains persist in the macronucleus (co-maintainers). We report here that long-term culture of co-maintainers has yielded recombinant rDNA molecules combining sequences from both parental inbred strains. The genetic structure of such molecules also gives us virtual certainty that the differential replication of C3 rDNA with respect to B rDNA is due to the DNA sequence difference previously reported in domain 2 of the rDNA replication regions of the two strains.