Retrotransposons can facilitate repair of broken chromosomes, and therefore an important question is whether the host can activate retrotransposons in response to chromosomal lesions. Here we show that Ty1 elements, which are LTR-retrotransposons in Saccharomyces cerevisiae, are mobilized when DNA lesions are created by the loss of telomere function. Inactivation of telomerase in yeast results in progressive shortening of telomeric DNA, eventually triggering a DNA-damage checkpoint that arrests cells in G 2͞M. A fraction of cells, termed survivors, recover from arrest by forming alternative telomere structures. When telomerase is inactivated, Ty1 retrotransposition increases substantially in parallel with telomere erosion and then partially declines when survivors emerge. Retrotransposition is stimulated at the level of Ty1 cDNA synthesis, causing cDNA levels to increase 20-fold or more before survivors form. This response is elicited through a signaling pathway that includes Rad24, Rad17, and Rad9, three components of the DNAdamage checkpoint. Our findings indicate that Ty1 retrotransposons are activated as part of the cellular response to telomere dysfunction.

The yeast LTR retrotransposon Ty1 integrates preferentially into regions upstream of tRNA genes. The chromatin structure of transcriptionally active tRNA genes is known to be important for Ty1 integration, but specific chromatin factors that enhance integration at tRNA genes have not been identified. Here we report that the histone deacetylase, Hos2, and the Trithorax-group protein, Set3, both components of the Set3 complex (Set3C), enhance transposition of chromosomal Ty1 elements by promoting integration into the upstream region of tRNA genes. Deletion of HOS2 or SET3 reduced the mobility of a chromosomal Ty1his3AI element about sevenfold. Despite the fact that Ty1his3AI RNA, total Ty1 RNA, and total Ty1 cDNA levels were not reduced in hos2D or set3D mutants, transposition of endogenous Ty1 elements into the upstream regions of tRNAGly genes was substantially decreased. Furthermore, when equivalent numbers of Ty1HIS3 mobility events launched from a pGAL1:Ty1his3AI plasmid were analyzed, only one-quarter to one-half as many were found upstream of tRNAGly genes in a hos2D or set3D mutant than in a wild-type strain. Chromatin immunoprecipitation analysis revealed that Hos2 is physically associated with tRNA genes. Taken together, our results support the hypothesis that Hos2 and Set3 function at tRNA genes to promote Ty1 integration.

Sox10 is a member of the group E Sox transcription factor family and plays key roles in neural crest development and subsequent cellular differentiation. Sox10 binds to regulatory sequences in target genes via its conserved high mobility group domain. In most cases, Sox10 exerts its transcriptional effects in concert with other DNA-binding factors, adaptor proteins, and nuclear import proteins. These interactions can lead to synergistic gene activation and can be cell type-specific. In earlier work, we demonstrated that Sox10 transactivates the nicotinic acetylcholine receptor ␣3 and ␤4 subunit genes and does so only in neuronal-like cell lines, raising the possibility that Sox10 mediates its effects via interactions with co-regulatory factors. Here we describe the identification of the armadillo repeat-containing protein, ARMCX3, as a Sox10-interacting protein. Biochemical analyses indicate that ARMCX3 is an integral membrane protein of the mitochondrial outer membrane. Others have shown that Sox10 is a nucleocytoplasmic shuttling protein. We extend this observation and demonstrate that, in the cytoplasm, Sox10 is peripherally associated with the mitochondrial outer membrane. Both Sox10 and ARMCX3 are expressed in mouse brain and spinal cord as well as several cell lines. Overexpression of ARMCX3 increased the amount of mitochondrially associated Sox10. In addition, although ARMCX3 does not possess intrinsic transcriptional activity, it does enhance transactivation of the nicotinic acetylcholine receptor ␣3 and ␤4 subunit gene promoters by Sox10. These results suggest that Sox10 is a membrane-associated factor whose transcriptional function is increased by direct interactions with ARMCX3 and raise the possibility of a signal transduction cascade between the nucleus and mitochondria through Sox10/ARMCX3 interactions.