THE TRNA(TYR)-ISOACCEPTORS AND THEIR GENES IN THE CILIATE TETRAHYMENA-THERMOPHILA - CYTOPLASMIC TRNA(TYR) HAS A Q-PSI-A ANTICODON AND IS CODED BY MULTIPLE INTRON-CONTAINING GENES

In the ciliated protozoa Tetrahymena thermophila introns have been det ected in rRNA and mRNAs until now. We have isolated and sequenced seve n tRNA(Tyr) genes from the T.thermophila nuclear genome, All of these genes contain introns of identical length and sequence. The 11 bp long intervening sequences are located 1 nt 3' to the anticodon as found i n other eukaryotic nuclear tRNA genes, Tetrahymena tRNA(Tyr) genes are efficiently transcribed in HeLa cell nuclear extract, Moreover, proce ssing and splicing occurred in HeLa as well as in wheat germ extracts, supporting the notion that Tetrahymena tRNA(Tyr) introns can be class ified as authentic tRNA introns, We have also isolated cytoplasmic tRN A(Tyr) from Tetrahymena cells. This tRNA(Tyr) isoacceptor has a Q psi A anticodon and is not a UAG suppressor as shown in in vitro translati on studies, Since UAG and UAA codons are used as glutamine codons in T etrahymena macronuclear DNA, the presence of a strong natural UAG supp ressor such as tRNA(Tyr) With G psi A anticodon should cause misreadin g of the glutamine as tyrosine codons and the absence of the latter ha d thus been predicted. Furthermore we have studied the organization of tRNA(Tyr) genes in the genome of T.thermophila and have found two typ es of tRNA(Tyr) gene arrangement, A minimum of 12 tRNA(Tyr) genes are present as single copies in genomic DNA HindIII restriction fragments ranging in size from 0.6 to 7 kb, Additionally one cluster of tRNA(Tyr ) genes consisting of six members has been detected in a 2.3 kb HindII I fragment.