A drug-resistant mutation in the ribosomal DNA of Tetrahymena.

A mutation that confers resistance to the drug paromomycin is shown to be in the structural gene that codes for the ribosomal RNA in Tetrahymena. This observation was made by exploiting a variant of the ribosomal DNA that distorts amplification of this locus when a new somatic nucleus develops during conjugation. Because the allelic forms of this locus have a restriction endonuclease site polymorphism, it was possible to correlate drug resistance with presence of a specific allele. The genetic results have been confirmed by sequence analysis (presented elsewhere). Thus, the crosses presented here provide a unique opportunity to identify mutations in the ribosomal DNA.