A dispersed tyrosine tRNA gene from Xenopus laevis with high transcriptional activity in vitro.

Several kinds of tRNA genes of Xenopus laevis are clustered together within tandemly repeated 3.18-kilobase DNA fragments. Other members of these reiterated tRNA gene families are dispersed and irregularly arranged in the genome. Here we report the isolation and some characteristics of one such dispersed gene that codes for a tyrosine tRNA. It is located within a low copy number 9.4-kilobase restriction fragment that contains no other RNA polymerase III gene functional in vitro. The dispersed gene differs from the clustered tyrosine tRNA gene by a single purine transition within the coding region, by extensive sequence differences within the intervening sequence and 5' and 3' flanking regions, and by its approximately 6-fold higher transcriptional activity in homologous S-100 extracts. Analyses of hybrid genes and deletion mutants demonstrate that this differential transcription is due to DNA in the 5' flanking regions.