Histones—Not Only a Static Skeleton

The topic of histone modifications has been reviewed recently (Elgin and Weintraub, 1975 Dixon, 1976 Allffey, 1977 Isenberg, 1979), and no attempt will be made here to give a comprehensive review. However, we will present a brief summary of results which can be correlated with the structural role of histones. 

The topic of histone acetylation has been the subject of extensive reviews (Dixon et al., 1975 Dixon, 1976 Allfrey, 1977 Isenberg, 1979). Acetylation occurs at specific lysine residues. In the case of the four core histones the acetylation sites are all located in the amino-terminal half of the molecule. 

Two types of modifications occur (see Figs. 1 and 2) (a) amino-terminal acetylation of histones HI, H2A, and H4 with the formation of a-acetylserine. This modification occurs in the cytoplasm before the histones are transported to the nucleus (Liew et al., 1970), and is apparently an irreversible modification (b) the formation of N8- 

The site of acetylation in the amino-terminal region has prompted suggestions to explain how such modifications might modulate histone binding to the DNA (for references, see Allfrey, 1971, 1977  

Dixon et al., 1975 Dixon, 1976). However, there is still not enough experimental evidence to support any one particular model. 

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