<P> HDAC11 has been shown to be related to HDACs 3 and 8, but its overall sequence is quite different from the other HDACs, leading it to be in its own category . HDAC11 has a catalytic domain located in its N - terminus . It has not been found incorporated in any HDAC complexes such as Nurd or SMRT which means it may have a special function unique to itself . It has been found that HDAC11 remains mainly in the nucleus . </P> <P> The discovery of histone acetylation causing changes in transcription activity can be traced back to the work of Vicent Allfrey and colleagues in 1964 . The group hypothesized that histone proteins modified by acetyl groups added negative charges to the positive lysines, and thus, reduced the interaction between DNA and histones . Histone modification is now considered a major regulatory mechanism that is involved in many different stages of genetic functions . Our current understanding is that acetylated lysine residues on histone tails is associated with transcriptional activation . In turn, deacetylated histones are associated with transcriptional repression . In addition, negative correlations have been found between several histone acetylation marks . </P> <P> The regulatory mechanism is thought to be twofold . Lysine is an amino acid with a positive charge when unmodified . Lysines on the amino terminal tails of histones have a tendency to weaken the chromatin's overall structure . Addition of an acetyl group, which carries a negative charge, effectively removes the positive charge and hence, reduces the interaction between the histone tail and the nucleosome . This opens up the usually tightly packed nucleosome and allows transcription machinery to come into contact with the DNA template, leading to gene transcription . Repression of gene transcription is achieved by the reverse of this mechanism . The acetyl group is removed by one of the HDAC enzymes during deacetylation, allowing histones to interact with DNA more tightly to form compacted nucleosome assembly . This increase in the rigid structure prevents the incorporation of transcriptional machinery, effectively silencing gene transcription . </P> <P> Another implication of histone acetylation is to provide a platform for protein binding . As a posttranslational modification, the acetylation of histones can attract proteins to elongated chromatin that has been marked by acetyl groups . It has been hypothesized that the histone tails offer recognition sites that attract proteins responsible for transcriptional activation . Unlike histone core proteins, histone tails are not part of the nucleosome core and are exposed to protein interaction . A model proposed that the acetylation of H3 histones activates gene transcription by attracting other transcription related complexes . Therefore, the acetyl mark provides a site for protein recognition where transcription factors interact with the acetylated histone tails via their bromodomain . </P>

What role does histone tail acetylation play in regulating gene expression