THE 2 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 th eir corresponding cDNAs. Our results reveal that the two genes (gamma- tub 1 and gamma-tub 2) have introns in homologous positions, but diffe r 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 fo r polypeptides of 462 amino acids (aa). The two genes share 76% identi ty 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 oth er Euplotes genes, probably code for cysteines. No UGA triplet was fou nd in the gamma-tub 1 gene. Further studies on the cDNA ends indicate that gamma-tub 1 uses at least three transcription initiation sites an d two poly(A) addition sites. In contrast, only one transcription init iation site and one poly(A) addition site were identified in gamma-tub 2. (C) 1998 Elsevier Science B.V.