Role of SAGA in the asymmetric segregation of DNA circles during yeast ageing.

Annina Denoth-Lippuner,Marek Konrad Krzyzanowski,Catherine Stober,Yves Barral

doi:10.7554/elife.03790

Abstract

In eukaryotes, intra-chromosomal recombination generates DNA circles, but little is known about how cells react to them. In yeast, partitioning of such circles to the mother cell at mitosis ensures their loss from the population but promotes replicative ageing. Nevertheless, the mechanisms of partitioning are debated. In this study, we show that the SAGA complex mediates the interaction of non-chromosomal DNA circles with nuclear pore complexes (NPCs) and thereby promotes their confinement in the mother cell. Reciprocally, this causes retention and accumulation of NPCs, which affects the organization of ageing nuclei. Thus, SAGA prevents the spreading of DNA circles by linking them to NPCs, but unavoidably causes accumulation of circles and NPCs in the mother cell, and thereby promotes ageing. Together, our data provide a unifying model for the asymmetric segregation of DNA circles and how age affects nuclear organization.

Highlights

Homologous recombination plays an important role in DNA repair in all organisms and acts through the formation of a Holliday junction between the damaged molecule and its repair template
In 38% (±6%) of the wild-type cells, all extrachromosomal rDNA circles (ERCs) that had accumulated in the mother cell remained in the mother cell, as demonstrated by the fact that their daughters lacked the ADE2 marker and formed a red colony (see material and methods, (Sinclair and Guarente, 1997)). This percentage was reduced 2.5-fold in the gcn5∆ and the sgf73∆ single mutant cells (16 ± 2% and 15 ± 2%, N ≥ 3 clones, p < 0.001; Figure 3B), indicating that high-fidelity retention of ERCs in the mother cell requires Spt-Ada-Gcn5 acetyltransferase (SAGA) function. To further test this conclusion, we investigated whether the accumulation of ERCs in aged yeast mother cells depended on SAGA function
Whereas the function of SAGA in chromatin anchorage to nuclear pore complexes (NPCs) was intriguing in this context, we considered that SAGA might contribute to the retention of non-chromosomal DNA circles indirectly through its role in transcription

Summary

Introduction

Homologous recombination plays an important role in DNA repair in all organisms and acts through the formation of a Holliday junction between the damaged molecule and its repair template. When recombination involves two homologous chromosomes, the resolution of the Holliday junction leads to a cross-over, the exchange of arms between two chromosomes, in 50% of the cases (Heyer, 2004). In 50% of these events, the resolution of the Holliday junction leads to the excision of a DNA circle containing one or several of the involved repeats (Gaubatz, 1990). Due to the highly repeated nature of the ribosomal DNA (rDNA) locus, rDNA circles have been found in all eukaryotes tested so far (Cohen et al, 2010). How cells react to these molecules is not understood. It is unclear whether these cells recognize, sort, and eliminate these molecules or let them passively disappear by dilution

Methods

Results

Conclusion