The Rad27 (Fen-1) nuclease inhibits Ty1 mobility in Saccharomyces cerevisiae

Although most Tyl elements in Saccharomyces cerevisiae are competent for retrotransposition, host defense genes can inhibit different steps of the Tyl life cycle. Here, we demonstrate that Rad27, a structure-specific nuclease that plays an important role in DNA replication and genome stability, inhibits Tyl at a post-translational level. We have examined the effects of various rad27 mutations on Tyl element retrotransposition and cDNA recombination, termed Tyl mobility. The point mutations rad27-G67S, rad27-G240D, and rad27-E158D that cause defects in certain enzymatic activities in vitro result in variable increases in Tyl mobility, ranging from 4- to 22-fold. The C-terminal frameshift mutation rad27-324 confers the maximum increase in Tyl mobility (198-fold), unincorporated cDNA, and insertion at preferred target sites. The null mutation differs from the other rad27alleles by increasing the frequency of multimeric Tyl insertions and cDNA recombination with a genomic element. The rad27 mutants do not markedly alter the levels of Tyl RNA or the TyAl-gag protein. However, there is an increase in the stability of unincorporated Tyl cDNA in rad27-324 and the null mutant. Our results suggest that Rad27 inhibits Tyl mobility by destabilizing unincorporated Tyl cDNA and preventing the formation of Tyl multimers.

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