The DNA sequences of a cDNA clone and the macronuclear genomic fragment 
corresponding to the functional copy of the SerH3 surface antigen gene of 
Tetrahymena thermophila were determined. Primer extension and nuclease 
protection assays show that the SerH3 transcription unit is 1,425 nucleotides 
long and contains no introns. The predicted polypeptide encoded by the SerH3 
gene has a molecular mass of 44,415 daltons; one-third of its 439 residues are 
either cysteine, serine, or threonine. The central half of the polypeptide 
consists of three homologous domains in tandem array; within these domains, the 
cysteine, proline, and tryptophan residues occur in highly regular patterns.