Delta sequences in the 5' non-coding region of yeast tRNA genes

Two so far undetected tRNA genes were found close to delta (delta) sequences at the sup4 locus on chromosome X in the genome of Saccharomyces cerevisiae. The two genes were identified from their abundant transcription products in frog oocytes. Hybridisation experiments allowed the mapping of the transcripts in cloned DNA and DNA sequence analysis revealed the presence of one AGGtRNA and one GACtRNA gene. tRNA genes with sequences similar or identical to GACtRNA exist in 14-16 copies per haploid yeast genome, whereas only one copy was detected for AGGtRNA. In vivo labelling of total yeast tRNA with P followed by hybridisation revealed that the unique AGGtRNA gene is transcribed in S. cerevisiae. delta sequences are present 120 bp upstream from the first coding nucleotide in the case of AGGtRNA, 80 bp in the case of GACtRNA and 405 bp in the case of the known UACtRNA (sup4) gene. delta sequences, as part of Ty elements or alone, were also found by other investigators at similar distances upstream of the mRNA start in mutant alleles of protein-coding yeast genes. Although protein-coding genes are transcribed by RNA polymerase II and tRNA genes by RNA polymerase III, the 5' non-coding region of both types of genes could conceivably have a peculiar DNA or chromatin structure used as preferred landing sites by transposable elements.