The two gamma-tubulin-encoding genes of the ciliate Euplotes crassus differ in their sequences, codon usage, transcription initiation sites and poly(A) addition sites.

We have isolated and sequenced two gamma-tubulin (gamma-Tub)-encoding macronuclear genes of the ciliate Euplotes crassus (Ec), as well as their corresponding cDNAs. Our results reveal that the two genes (gamma-tub 1 and gamma-tub 2) have introns in homologous positions, but differ in their sequences, codon usage, transcription initiation sites and poly(A) addition sites. They both consist of three exons, two introns and two short non-coding sequences on both ends, and they both code for polypeptides of 462 amino acids (aa). The two genes share 76% identity at the nucleotide (nt) level, 86% at the deduced aa level and show 61-92% aa homology to the gamma-Tubs of other organisms. The gamma-tub 2 gene contains two in-frame UGA codons which, like UGA codons in other Euplotes genes, probably code for cysteines. No UGA triplet was found in the gamma-tub 1 gene. Further studies on the cDNA ends indicate that gamma-tub 1 uses at least three transcription initiation sites and two poly(A) addition sites. In contrast, only one transcription initiation site and one poly(A) addition site were identified in gamma-tub 2.