Heterochromatin boundaries

Chromosomes are composed of functionally distinct domains separated by chromatin boundaries. In the August 10 Science, Noma et al. define the boundaries of the heterochromatin domain at the mating-type locus (mat) in fission yeast (Science 2001, 293:1150-1155). They used chromatin immunoprecipitation (ChIP) and PCR analysis to scan the entire mat region (about 47 kilobases). Methylation of lysine 9 of histone H3 and the Swi6 protein were both enriched in a 20kb interval that contains the mat2/3 silent cassettes. The inverted repeats IR-L and IR-R mark the boundaries of this heterochromatin domain. In contrast, methylation of lysine 4 of histone H3 showed an inverse pattern and was excluded from the mat2/3 region. Noma et al. show that deletion of the boundary IR-L and IR-R repeats resulted in spreading of both H3-Lys9 methylation and Swi6 beyond the normal heterochromatin domain (up to 8 kilobases). Thus, distinct H3 methylation patterns define euchromatic (H3-Lys4-Methyl) and heterochromatic (H3-Lys9-Methyl) domains. The boundaries of these domains are dictated by the presence of insulating elements.