Table 1. Transcription-related modifications of lysine residues in histone proteins.

Histone modification	Modifying enzymes	Demodifying enzymes	Primary transcriptional effect	Nature of modification
Acetylation	HATs	HDACs	Activation	Dynamic
Methylation	HMTs	???	Activation/repression	Static
Ubiquitylation	Rad6*	Ubiquitin proteases (UBPs)	Activation	Dynamic
Sumoylation	UBC9*	ULP-related proteases†	Repression	Dynamic

^*Although other enzymes are involved in sumoylation and ubiquitylation, UBC9 (yeast through human) is the only known E2 SUMO-conjugating enzyme, and yeast Rad6 is the ubiquitin E2 known to monoubiquitylate histones in vivo

^†Although histone desumoylation has not yet been characterized, studies with other sumoylated substrates demonstrate deconjugation by ULP proteases