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

摘要

In the ciliated protozoa Tetrahymena thermophila introns have been detected in rRNA and mRNAs until now. We have isolated and sequenced seven 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 in other eukaryotic nuclear tRNA genes, Tetrahymena tRNA(Tyr) genes are efficiently transcribed in HeLa cell nuclear extract, Moreover, processing and splicing occurred in HeLa as well as in wheat germ extracts, supporting the notion that Tetrahymena tRNA(Tyr) introns can be classified as authentic tRNA introns, We have also isolated cytoplasmic tRNA(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 translation studies, Since UAG and UAA codons are used as glutamine codons in Tetrahymena macronuclear DNA, the presence of a strong natural UAG suppressor such as tRNA(Tyr) With G psi A anticodon should cause misreading of the glutamine as tyrosine codons and the absence of the latter had thus been predicted. Furthermore we have studied the organization of tRNA(Tyr) genes in the genome of T.thermophila and have found two types 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 HindIII fragment.