N. Ke et Df. Voytas, cDNA of the yeast retrotransposon Ty5 preferentially recombines with substrates in silent chromatin, MOL CELL B, 19(1), 1999, pp. 484-494
The yeast retrotransposon Ty5 preferentially integrates into regions of sil
ent chromatin. Ty5 cDNA also recombines with homologous sequences, generati
ng tandem elements or elements that have exchanged markers between cDNA and
substrate. In this study, we demonstrate that Ty5 integration depends upon
the conserved DD(35)E domain of integrase and cis-acting sequences at the
end of the long terminal repeat (LTR) implicated in integrase binding, cDNA
recombination requires Rad52p, which is responsible for homologous recombi
nation. Interestingly, Ty5 cDNA recombines at least three times more freque
ntly with substrates in silent chromatin than with a control substrate at a
n internal chromosomal locus. This preference depends upon the Ty5 targetin
g domain that is responsible for integration specificity, suggesting that l
ocalization of cDNA to silent chromatin results in the enhanced recombinati
on. Recombination with a telomeric substrate occasionally generates highly
reiterated Ty5 arrays, and mechanisms for tandem element formation were exp
lored by using a plasmid based recombination assay. Point mutations were in
troduced into plasmid targets, and recombination products were characterize
d to determine recombination initiation sites. Despite our previous observa
tion of the importance of the LTR in forming tandem elements, recombination
cannot simply be explained by crossover events between the LTRs of substra
te and cDNA. We propose an alternative model based on single-strand anneali
ng, where single-stranded cDNA initiates tandem element formation and the L
TR is required for strand displacement to form a looped intermediate. Retro
transposons are increasingly found associated with chromosome ends, and amp
lification of Ty5 by both integration and recombination exemplifies how ret
roelements can contribute to telomere dynamics.