INITIATOR METHIONINE TRANSFER-RNA IS ESSENTIAL FOR TY1 TRANSPOSITION IN YEAST

The yeast retrotransposon Tyl transposes through an RNA intermediate by a mechanism similar to that of retroviral reverse transcription and integration. Tyl 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 t is used as a primer for Tyl reverse transcription. Mutations in the Tyl element that alter 5 of 10 nucleotides that are complementary to the tRNA(i)Met abolish Tyl transposition, even though they are silent with regard to Tyl 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 Tyl -PBS mutant. The compensatory mutations made in the tRNA(i)Met alleviate the transposition defect of the Tyl -PBS mutant. The mutant and wild-type tRNA(i)Met are enriched within Tyl virus-like particles irrespective of complementarity to the Tyl -PBS. Thus, complementarity between the Tyl -PBS and tRNA(i)Met is essential for transposition but is not necessary for packaging of the tRNA inside virus-like particles.