Initiator methionine tRNA is essential for Ty1 transposition in yeast

The yeast retrotransposon Ty1 transposes through an RNA intermediate by a mechanism similar to that of retroviral reverse transcription and integration. Ty1 RNA contains a putative minus strand primer binding site (-PBS) that is complementary to the 3′ acceptor stem of the initiator methionine tRNA (tRNA_i^Met). Here we demonstrate that the tRNA_i^Met is used as a primer for Ty1 reverse transcription. Mutations in the Ty1 element that alter 5 of 10 nucleotides that are complementary to the tRNA_i^Met abolish Ty1 transposition, even though they are silent with regard to Ty1 protein coding. We have constructed a yeast strain lacking wild-type tRNA_i^Met that is dependent on a mutant derivative of tRNA_i^Met that has an altered acceptor stem sequence, engineered to restore homology with the Ty1 -PBS mutant. The compensatory mutations made in the tRNA_i^Met alleviate the transposition defect of the Ty1 -PBS mutant. The mutant and wild-type tRNA_i^Met are enriched within Ty1 virus-like particles irrespective of complementarity to the Ty1 -PBS. Thus, complementarity between the Ty1 -PBS and tRNA_i^Met is essential for transposition but is not necessary for packaging of the tRNA inside virus-like particles.