The aminoacceptor stem of the yeast tRNA(Lys) contains determinants of mitochondrial import selectivity.

Yeast tRNA(Lys)CUU is nucleus-encoded and is partially imported into the mitochondria. Another lysine isoacceptor, tRNA(Lys)SUU, is also nucleus-encoded but is not imported. These two tRNAs differ in 21 bases. We have previously localised import selectivity determinants in the anticodon arm. By in vitro import of mutant transcripts and by expression of mutant tRNA genes in vivo we show here that the first base pair (1:72) and the discriminator base 73 are also relevant to import selectivity. Replacement of bases 1:72 in tRNA(Lys)SUU by those of tRNA(Lys)CUU makes it importable with a transport efficiency similar to natural.