Extrachromosomal rDNA circle
Extrachromosomal rDNA circles are pieces of extrachromosomal circular DNA derived from ribosomal DNA. Initially found in baker's yeast, these self-replicating circles are suggested to contribute to their aging and found in their aged cells. Like ordinary eccDNA, they are created by intra-molecular homologous recombination of the chromosome. The process for intra-molecular homologous recombination is independent of chromosomal replication. The de novo generated circles had exact multiples of tandem copies of 2-kb fragments from cosmid templates. The tandem organization is essential to circle formation. Looping out of organized ribosomal genes in intergenic nontranscribed spacers yielded either large or small repeat circles dependent on large or short repeats of the spacer.
In yeast strains
The Sgs1 gene mutations in yeast mother cells were shown to have accelerated aging, suggesting their function to cellular senescence. ERCs accumulate in old cells and mutations of Sgs1 were found to increase this accumulation, leading to the idea that ERCs lead to shorter lifespan of cells. Vice versa, deletion of Fob1 slows down ERC accumulation and increases lifespan. Accumulated ERCs impair cell proliferation in old cells by interfering with the expression of important cell cycle genes at the G1/S transition and thus delay cell cycle progression. The ERC accumulate in the mother cell during the budding process. Sinclair et al. mentioned a suggested common mechanism between the Sgs1 and WRN genes since they both had age related effects on yeast and human aging respectively.Borghouts et al., resolved the two mechanisms, retrograde response and the increase in cellular content of ERCs, that affected longevity in yeast. They determined that the generation of ERCs negatively influence the life spans of grande and petite yeast strains.