DNA rearrangements associated with the H3 surface antigen gene of Tetrahymena thermophila that occur during macronuclear development.

The surfaces of Tetrahymena thermophila cells grown between 20 and 35 degrees C are covered by one or more variants of H antigens. A cDNA clone, pC6, has previously been identified that hybridizes to a unique polyA+ RNA that appears to code for the SerH3 variant of the H antigens. pC6 and a subclone of it, pGpC6.295, were used to analyze the genomic organization of the corresponding gene(s) in both the macronucleus and the micronucleus. It was determined that pC6 hybridizes to a small family of sequences in the macronucleus, only one of which also hybridizes to pGpC6.295. The latter is a strong candidate for the gene encoding the SerH3 antigen. Sequences homologous to pC6 - but not to pGpC6.295 - are present in strains carrying the other SerH alleles. Shifts in antigen switching during vegetative growth do not result in any detectable DNA rearrangements in the vicinity of the pC6-hybridizing sequence family. Analysis of micronuclear DNA from a homozygous SerH3 strain revealed that it also contains a family of sequences that are homologous to pC6; but, in contrast to the macronuclear DNA, two members of this micronuclear sequence family hybridize to pGpC6.295. Comparison of micro- and macronuclear DNA indicate that some members of the pC6-positive sequence family rearrange during macronuclear development. These rearrangements fall into two classes: those which occur reproducibly, and those which show variability. The gene homologous to pGp6.295 falls into the former category.