Histone modifications modulate the packing of chromatin.
The level of packing of the DNA is important for gene transcription, since the transcriptional machinery must have access to the promoter in order for transcription to occur.
 Neutralization of charged lysine residues by HATs allows for the chromatin to decondense so that this machinery has access to the gene to be transcribed.
However, acetylation is not always associated with enhanced transcriptional activity.
For instance, acetylation of H4K12 has been associated with condensed and transcriptionally inactive chromatin.
In addition, some histone modifications are associated with both enhanced and repressed activity, in a context-dependent manner.

HATs act as transcriptional co-activators or gene silencers and are most often found in large complexes made up of 10 to 20 subunits, some of which shared among different HAT complexes.
 These complexes include SAGA (Spt/Ada/Gcn5L acetyltransferase), PCAF, ADA (transcriptional adaptor), TFIID (transcription factor II D), TFTC (TBP-free TAF-containing complex), and NuA3/NuA4 (nucleosomal acetyltransferases of H3 and H4).
 These complexes modulate HAT specificity by bringing HATs to their target genes where they can then acetylate nucleosomal histones.
 Some HAT transcriptional co-activators contain a bromodomain, a 110-amino acid module that recognizes acetylated lysine residues and is functionally linked to the co-activators in the regulation of transcription.
Histone modifications modulate the packing of chromatin. The level of packing of the DNA is important for gene transcription, since the transcriptional machinery must have access to the promoter in order for transcription to occur.  Neutralization of charged lysine residues by HATs allows for the chromatin to decondense so that this machinery has access to the gene to be transcribed. However, acetylation is not always associated with enhanced transcriptional activity. For instance, acetylation of H4K12 has been associated with condensed and transcriptionally inactive chromatin. In addition, some histone modifications are associated with both enhanced and repressed activity, in a context-dependent manner.
HATs act as transcriptional co-activators or gene silencers and are most often found in large complexes made up of 10 to 20 subunits, some of which shared among different HAT complexes.  These complexes include SAGA (Spt/Ada/Gcn5L acetyltransferase), PCAF, ADA (transcriptional adaptor), TFIID (transcription factor II D), TFTC (TBP-free TAF-containing complex), and NuA3/NuA4 (nucleosomal acetyltransferases of H3 and H4).  These complexes modulate HAT specificity by bringing HATs to their target genes where they can then acetylate nucleosomal histones.  Some HAT transcriptional co-activators contain a bromodomain, a 110-amino acid module that recognizes acetylated lysine residues and is functionally linked to the co-activators in the regulation of transcription.
Histone acetyltransferase