The tRNATyr-isoacceptors and their genes in the ciliate Tetrahymena thermophila: Cytoplasmic tRNATyr has a QΨA 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 tRNATyr 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 tRNATyr 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 tRNATyr introns can be classified as authentic tRNA introns. We have also isolated cytoplasmic tRNATyr from Tetrahymena cells. This tRNATyr isoacceptor has a QΨA 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 tRNATyr with GΨA 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 tRNATyr genes in the genome of T.thermophila and have found two types of tRNATyr gene arrangement. A minimum of 12 tRNATyr 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 tRNATyr genes consisting of six members has been detected in a 2.3 kb HindIII fragment.