A human tRNA gene heterocluster encoding threonine, proline and valine tRNAs

A cluster of three tRNA genes encoding a tRNA^Thr_UGU' a tRNA^Pro_UGG', and atRNA^Val_AAC' and two Alu-elements occur in a 6.0-kb human DNA fragment. The tRNA^Thr gene is 2.7-kb upstream from the tRNA^Pro gene, which is separated by 367 bp from the tRNA^Val gene. One Alu-element actually overlaps the tRNA^Val gene and is of opposite polarity to all three tRNA genes. All three tRNA genes are accurately transcribed in a homologous HeLa cell extract, since the ribonuclease T₁ fingerprints of the tRNA transcripts are consistent with the nucleotide sequences of the tRNAs. The upstream region flanking the tRNA^Thr gene has two tracts of alternating purine/pyrimidine residues potentially capable of adopting the Z-DNA conformation, and presumptive binding sites for two RNA polymerase II transcription factors. The tRNA^Thr gene apparently has a substantially higher in vitro transcriptional efficiency than the other two tRNA genes in this cluster, and a tRNA^Gly_GCC gene from another human DNA segment. Deletion constructs of the tRNA^Thr gene retaining 272, 168, and 33 bp of original 5'-flanking DNA had about the same in vitro transcriptional efficiency, whereas that of the construct with only 2bp of 5'-flanking human DNA was drastically reduced. The tRNA^Thr gene constructs with 272 and 168 bp of original 5'-flanking DNA apparently reduce the transcriptional efficiencies of the proline and glycine tRNA genes, implicating the upstream region from the tRNA^Thr gene as being crucial for its high transcriptional efficiency.