The budding yeast Saccharomyces cerevisiae
has become a favorite model for studying the genetic and molecular basis for variation in lifespan 
. In particular, the unequal division of mother and daughter cells in this species makes it especially amenable for analysis of replicative lifespan (RLS), the number of “daughter” cells that a “mother” cell can produce before it senesces, which is typically 20–30 
. RLS is thought to be analogous to aging of mitotic cells such as stem cells and epithelia. Genetic screens for RLS alteration have identified many genes whose deletion confers lifespan extension, such as those in growth and metabolism pathways, and even more genes whose absence reduces yeast longevity 
The discovery that long-lived mutants redirect the SIR (Silent Information Regulator) complex, including the NAD-dependent histone deacetylase Sir2, to the nucleolus first implicated events at the rDNA as potential determinant of aging 
. The identification of the rDNA binding protein Fob1 as a regulator of yeast lifespan further supported rDNA's involvement in aging 
. The rDNA locus is a highly repetitive region of the genome, over 1MB in size, consisting of approximately 150 tandem repeats of a 9.1 kb sequence that encode the 35S transcription unit (18S, 5.8S, 25S RNAs transcribed by RNA polymerase I) and the 5S gene (transcribed by RNA polymerase III). More than 60% of all transcription in a yeast cell is dedicated to the production of these rRNAs 
, which can be accomplished only through the concurrent transcription from multiple rDNA repeats 
. Each repeat also contains non-transcribed regions with features that allow it to maintain its chromosome-like size, such as origins of replication. An RNA polymerase II promoter (E-pro) is also found in one of the ‘non-transcribed” regions. It is this promoter whose activity is suppressed by Sir2. In the absence of Sir2, RNA polymerase II transcription disrupts rDNA-bound cohesin 
leading to an increase in intra-chromatid mitotic recombination and the formation of extrachromosomal rDNA circles (ERCs) 
. ERCs have been shown to preferentially segregate to mother cells and their accumulation in old mother cells has been proposed as a cause of aging in yeast, perhaps through sequestration of as yet unknown detrimental factors 
. An attractive feature of the ERC model is that it could explain how daughters can be rejuvenated, i.e. how daughters can have the same RLS regardless of whether they came from old or young mothers. Accumulation of ERCs in old mothers has been proposed to be responsible for the decreased lifespan seen in sir2Δ
and, conversely, the increased lifespan of strains that contain an extra genomic copy of SIR2
. The Fob1 protein binds to the rDNA replication fork barrier (RFB), creating a unidirectional replication block thought to prevent the head-on collision between the rRNA Polymerase I transcription machinery and the DNA replication machinery 
. However, Fob1 binding also promotes rDNA recombination by increasing the probability of double strand breaks and hence promoting ERC formation. As expected for a gene whose deletion decreases ERC accumulation, fob1Δ
mutants have increased RLS.
Calorie restriction is the best-studied intervention that promotes longevity 
. Restriction of dietary intake to 70% ad libitum
was first reported to extend the lifespans of mice and rats 
. Since then, calorie restriction-mediated lifespan extension has also been documented in yeast, worms, flies, and several other organisms 
. Significant progress has been achieved in dissecting the nutrient responsive signaling pathways responsible for this lifespan extension, some of which, like the TOR pathway, are shared between species as distant as mice and yeast. The downstream effectors of these signaling pathways, on the other hand, are unknown. It is known, however, that Sir2 and Fob1 act in a pathway that is genetically distinct from calorie restriction and that calorie restriction is able to extend the lifespan of sir2Δ fob1Δ
double mutants 
. The mechanism through which calorie extension extends lifespan is therefore not via direct regulation of Sir2 or Fob1 and remains mysterious.
To begin to address these gaps in our knowledge we used an outbred yeast model, consisting of strains generated in a cross between the S288c laboratory strain BY4716 (BY) and the vineyard strain RM11-1a (RM), to identify naturally occurring polymorphisms that regulate RLS and found that the rDNA locus is the major regulator of lifespan in this cross. Among the rDNA sequences that differ between RM and BY, we identified a polymorphism in the RM rDNA locus that leads to a marked reduction in rDNA origin activity both in a plasmid assay and at its native location in the genome. The less active RM rDNA origin confers a reduced size of the rDNA array and extends lifespan. Critically, this lifespan extension is independent of both Sir2 and Fob1 but the activity of both of these rDNA origins is strikingly affected by calorie restriction, thus suggesting a replication-based mechanism for calorie restriction-mediated extension of RLS.