FIGURE 8-29 Model for the formation of heterochromatin by the binding of HP1 to histone H3 trimethylated at lysine 9. (a) HP1 contributes to the condensation of heterochromatin by binding to histone H3 N-terminal tails trimethylated at lysine 9, then associating with other histone-bound HP1 molecules. (b) Heterochromatin condensation can spread along a chromosome because HP1 binds the histone methyltransferase (HMT) that methylates lysine 9 of histone H3. This creates a binding site for HP1 on the neighboring nucleosome. The spreading process continues until a “boundary element” is encountered. See G. Thiel et al., 2004, Eur. J. Biochem. 271:2855, and A. J. Bannister et al., 2001, Nature 410:120.