A RARE TRANSFER RNA-ARG(CCU) THAT REGULATES TY1 ELEMENT RIBOSOMAL FRAMESHIFTING IS ESSENTIAL FOR TY1 RETROTRANSPOSITION IN SACCHAROMYCES-CEREVISIAE

Translation of the yeast retrotransposon Tyl TYA1(gag)-TYB1(pol) gene occurs by a + 1 ribosomal frameshifting event at the sequence CUU AGG C. Because overexpression of a low abundance tRNA-Arg(CCU) encoded by the HSX1 gene resulted in a reduction in Ty1 frameshifting, it was sug gested that a translational pause at the AGG-Arg codon is required for optimum frameshifting. The present work shows that the absence of tRN A-Arg(CCU) affects Ty1 transposition, translational frameshifting, and accumulation of mature TYB1 proteins. Transposition of genetically ta gged Ty1 elements decreases at least 50-fold and translational framesh ifting increases 3-17-fold in cells lacking tRNA-Arg(CCU). Accumulatio n of Ty1-integrase and Ty1-reverse transcriptase/ribonuclease H is def ective in an hsx1 mutant. The defect in Ty1 transposition is complemen ted by the wild-type HSX1 gene or a mutant tRNA-Arg(UCU) gene containi ng a C for T substitution in the first position of the anticodon. Over expression of TYA 1 stimulates Ty 1 transposition 50-fold above wild-t ype levels when the level of transposition is compared in isogenic hsx 1 and HSX1 strains. Thus, the HSX1 gene determines the ratio of the TY A 1 to TYA 1 -TYB 1 precursors required for protein processing or stab ility, and keeps expression of TYB1 a rate-limiting step in the retrot ransposition cycle.