Mammalian telomeres have heterochromatic features, including trimethylated 
histone H3 at lysine 9 (H3K9me3) and trimethylated histone H4 at lysine 20 
(H4K20me3). In addition, subtelomeric DNA is hypermethylated. The enzymatic 
activities responsible for these modifications at telomeres are beginning to be 
characterized. In particular, H4K20me3 at telomeres could be catalyzed by the 
novel Suv4-20h1 and Suv4-20h2 histone methyltransferases (HMTases). In this 
study, we demonstrate that the Suv4-20h enzymes are responsible for this histone 
modification at telomeres. Cells deficient for Suv4-20h2 or for both Suv4-20h1 
and Suv4-20h2 show decreased levels of H4K20me3 at telomeres and subtelomeres in 
the absence of changes in H3K9me3. These epigenetic alterations are accompanied 
by telomere elongation, indicating a role for Suv4-20h HMTases in telomere 
length control. Finally, cells lacking either the Suv4-20h or Suv39h HMTases 
show increased frequencies of telomere recombination in the absence of changes 
in subtelomeric DNA methylation. These results demonstrate the importance of 
chromatin architecture in the maintenance of telomere length homeostasis and 
reveal a novel role for histone lysine methylation in controlling telomere 
recombination.