A mitochondrial mutator plasmid that causes senescence under dietary restricted conditions

BACKGROUND: Calorie or dietary restriction extends life span in a wide range of organisms including the filamentous fungus Podospora anserina. Under dietary restricted conditions, P. anserina isolates are several-fold longer lived. This is however not the case in isolates that carry one of the pAL2-1 homologous mitochondrial plasmids. RESULTS: We show that the pAL2-1 homologues act as 'insertional mutators' of the mitochondrial genome, which may explain their negative effect on life span extension. Sequencing revealed at least fourteen unique plasmid integration sites, of which twelve were located within the mitochondrial genome and two within copies of the plasmid itself. The plasmids were able to integrate in their entirety, via a non-homologous mode of recombination. Some of the integrated plasmid copies were truncated, which probably resulted from secondary, post-integrative, recombination processes. Integration sites were predominantly located within and surrounding the region containing the mitochondrial rDNA loci. CONCLUSION: We propose a model for the mechanism of integration, based on innate modes of mtDNA recombination, and discuss its possible link with the plasmid's negative effect on dietary restriction mediated life span extension

Yeast retrotransposons.

In the decade since Ty elements were discovered, advocates have argued they could be used as a genetic entrée to elusive host-type functions required by retroviruses. However, the advent of the polymerase chain reaction, coupled with a boom in funding for human immunodeficiency virus research have moved retroviral research apace, raising questions as to whether novel contributions would be realized. The past year, with the implication of the cell cycle and specific host proteins, such as the debranching enzyme and transcription initiation factors, in Ty retrotransposition has provided a positive answer and raised new questions

Integration of the yeast retrovirus-like element Ty3 upstream of a human tRNA gene expressed in yeast.

The retrovirus-like element Ty3 of Saccharomyces cerevisae integrates into the yeast genomic DNA in a position specific manner. Ty3 integrates within 1-2 base pairs of the site of transcription initiation by RNA polymerase III. The human tRNA(Lys)3 gene was used as a target for transposition in a plasmid-based assay to determine whether Ty3 integration can be targeted to a human tRNA gene. Each transposition event observed was adjacent to the site of initiation of transcription of the human tRNA gene. Therefore, heterologous tRNA genes can serve as targets for Ty3 in yeast. This is a first step toward development of a system for targeted integrations in heterologous organisms

Adjacent pol II and pol III promoters: transcription of the yeast retrotransposon Ty3 and a target tRNA gene.

The Saccharomyces cerevisiae retrotransposon Ty3 integrates 16 to 19 basepairs upstream of tRNA genes in a region where sequences have been shown to affect the expression of tRNA genes in vivo and in vitro. Sigma, the isolated long terminal repeat of Ty3, is also found in this region. The purpose of these experiments was to elucidate the effects of Ty3 and sigma expression on that of an associated SUP2 tRNA(Tyr) gene in vivo. SUP2 pre-tRNA levels were moderately increased when SUP2 was associated with Ty3 or sigma in either orientation. These increases were independent of Ty3 or sigma promoter activity. The presence of Ty3 or sigma also increased the usage of a minor SUP2 transcription initiation site 2 basepairs upstream of the major initiation site and within the 5 basepair direct repeat flanking Ty3 and sigma. Transcription from an isolated sigma directed toward the tRNA gene was observed to extend through the tRNA gene. In contrast to the lack of an effect of sigma induction on pre-tRNA(Tyr) levels, levels of this sigma transcript were increased when the SUP2 promoter was inactivated by a single basepair mutation