The evolutionarily-conserved sirtuin family of histone deacetylases regulates a 
multitude of DNA-associated processes. A recent genome-wide screen conducted in 
the yeast Saccharomyces cerevisiae identified Yku70/80, which regulate 
nonhomologous end-joining (NHEJ) and telomere structure, as being essential for 
cell proliferation in the presence of the pan-sirtuin inhibitor nicotinamide 
(NAM). Here, we show that sirtuin-dependent deacetylation of both histone H3 
lysine 56 and H4 lysine 16 promotes growth of yku70Δ and yku80Δ cells, and that 
the NAM sensitivity of these mutants is not caused by defects in DNA 
double-strand break repair by NHEJ, but rather by their inability to maintain 
normal telomere length. Indeed, our results indicate that in the absence of 
sirtuin activity, cells with abnormally short telomeres, e.g., yku70/80Δ or 
est1/2Δ mutants, present striking defects in S phase progression. Our data 
further suggest that early firing of replication origins at short telomeres 
compromises the cellular response to NAM- and genotoxin-induced replicative 
stress. Finally, we show that reducing H4K16ac in yku70Δ cells limits activation 
of the DNA damage checkpoint kinase Rad53 in response to replicative stress, 
which promotes usage of translesion synthesis and S phase progression. Our 
results reveal a novel interplay between sirtuin-mediated regulation of 
chromatin structure and telomere-regulating factors in promoting timely 
completion of S phase upon replicative stress.