DIFFERENTIAL EXPRESSION OF IMMOBILIZATION ANTIGEN GENES IN TETRAHYMENA-THERMOPHILA .1. GENETIC AND EPISTATIC RELATIONSHIPS AMONG RECESSIVE MUTATIONS WHICH ALTER NORMAL EXPRESSION OF I-ANTIGENS

Mutational analysis of the differentially expressed immobilization (i-) antigen genes of Tetrahymena thermophila shows that i-antigen expression is controlled by integrator-like genes that do not themselves specify i-antigens. This paper presents a more complete description of the r1 and r3 regulator loci (Doerder 1973) and describes a third locus, r4. It was found that homozygotes for mutations at each of these loci are deficient in the ability to express at least two of the i-antigen loci and that they express instead one of the alternate i-antigens. The i-antigen expressed by the r1 and r3 mutants, previously called r, is shown to be the i-antigen normally expressed when cells are grown in dilute homologous antiserum. A study of the epistatic relationships among the genes at these loci shows that the i-antigens themselves are probably not involved in their own regulation and that the previously proposed models to account for i-antigen expression are no longer valid. The data suggest that additional regulator loci must exist. © 1979 Springer-Verlag New York Inc.