The prevalent DNA modification in higher organisms is the methylation of cytosine to 5-methylcytosine (5mC), which is partially converted to 5-hydroxymethylcytosine (5hmC) by the Tet (ten eleven translocation) family of dioxygenases. Despite their importance in epigenetic regulation, it is unclear how these cytosine modifications are reversed. Here, we demonstrate that 5mC and 5hmC in DNA are oxidized to 5-carboxylcytosine (5caC) by Tet dioxygenases in vitro and in cultured cells. 5caC is specifically recognized and excised by thymine-DNA glycosylase (TDG). Depletion of TDG in mouse embyronic stem cells leads to accumulation of 5caC to a readily detectable level. These data suggest that oxidation of 5mC by Tet proteins followed by TDG-mediated base excision of 5caC constitutes a pathway for active DNA demethylation.

Acetylation of histone H3 lysine 56 (H3-K56) occurs in S phase, and cells lacking H3-K56 acetylation are sensitive to DNA-damaging agents. However, the histone acetyltransferase (HAT) that catalyzes global H3-K56 acetylation has not been found. Here we show that regulation of Ty1 transposition gene product 109 (Rtt109) is an H3-K56 HAT. Cells lacking Rtt109 or expressing rtt109 mutants with alterations at a conserved aspartate residue lose H3-K56 acetylation and exhibit increased sensitivity toward genotoxic agents, as well as elevated levels of spontaneous chromosome breaks. Thus, Rtt109, which shares no sequence homology with any other known HATs, is a unique HAT that acetylates H3-K56.

In Saccharomyces cerevisiae, known histone acetylation sites regulating gene activity are located in the N-terminal tails protruding from the nucleosome core. We report lysine 56 in histone H3 as a novel acetylation site that is located in the globular domain, where it extends toward the DNA major groove at the entry-exit points of the DNA superhelix as it wraps around the nucleosome. We show that K56 acetylation is enriched preferentially at certain active genes, such as those coding for histones. SPT10, a putative acetyltransferase, is required for cell cycle-specific K56 acetylation at histone genes. This allows recruitment of the nucleosome remodeling factor Snf5 and subsequent transcription. These findings indicate that histone H3 K56 acetylation at the entry-exit gate enables recruitment of the SWI/SNF nucleosome remodeling complex and so regulates gene activity.