FUNCTIONAL TRANSFER-RNAS WITH ALTERED 3' ENDS

The CCA trinucleotide is a universally conserved feature of the 3' end of tRNAs, where it serves as the site of amino acid attachment. Despite this extreme conservation, we have isolated functional mutants of tRNA(His) and tRNA1Val with altered CCA ends. A mutant that leads to de-repression of the histidine biosynthetic operon in Salmonella typhimurium has been characterized and found to have the CCA end of the sole tRNA(His) species mutated to UCA. However, constructed mutants of tRNA(His) with ACA or GCA ends appeared to be non-functional in vivo. Mutants of Escherichia coli tRNA1Val with GCA or ACA ends were isolated on the basis of their ability to promote frameshifting at a specific sequence. These same tRNA1Val mutants also caused read-through of stop codons that were one, or in some instances two, codons downstream of the valine codon decoded by the mutant tRNA. A startling implication of these data is that disruption of interactions between the CCA end of the tRNA and the large ribosomal subunit promotes these aberrant codon-anticodon interactions on the small ribosomal subunit.