A Human tRNA Gene Cluster Encoding the Major and Minor Valine tRNAs and a Lysine tRNA

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Abstract
A human genomic DNA clone hybridizing to mammalian valine tRNAIAC contained a cluster of three tRNA genes. Two valine tRNA genes with anticodons of AAC and CAC, encoding the major and minor cytoplasmic valine tRNA isoacceptors, respectively, and a lysine tRNACUU gene were identified by Southern blot hybridization and DNA sequence analysis of a 7.1-kb region. At least nine Alu family members were interspersed throughout the 18.5-kb human DNA fragment, with three Alu elements in the intergenic region between the valine tRNAAC gene and the lysine tRNA gene. Each of the five Alu family members in the sequenced region can be categorized into one of the four Alu subfamilies. The coding regions of all three tRNA genes contain characteristic internal split promoter sequences and typical RNA polymerase III termination signals in the 3′-flanking regions. The tRNA genes are accurately transcribed by RNA polymerase III in a HeLa cell extract, since the RNase T1 fingerprints of the mature-sized tRNA transcription products are consistent with the structural genes. The lysine tRNACUU gene was transcribed only slightly more efficiently than the valine tRNACAC gene in the homologous in vitro transcription system. Surprisingly, the valine tRNACAC gene was transcribed about eightfold more efficiently than the valine tRNAAC gene, implicating the presence of a modulatory element in the upstream region flanking the tRNACAC gene.