AN HP1-LIKE PROTEIN IS MISSING FROM TRANSCRIPTIONALLY SILENT MICRONUCLEI OF TETRAHYMENA

We report the identification and cloning of a 28-kDa polypeptide (p28) in Tetrahymena macronuclei that shares several features with the well studied heterochromatin-associated protein HP1 from Drosophila, Notab ly, like HP1, p28 contains both a chromodomain and a chromoshadow doma in. p28 also shares features with linker histone H1, and like H1, p28 is multiply phosphorylated, at least in part, by a proline-directed, C dc2-type kinase. As such, p28 is referred to as Hhp1p (for H1/HP1-like protein). Hhp1p is missing from transcriptionally silent micronuclei but is enriched in heterochromatin-like chromatin bodies that presumab ly comprise repressed chromatin in macronuclei. These findings shed li ght on the evolutionary conserved nature of heterochromatin in organis ms ranging from ciliates to humans and provide further evidence that H P1-like proteins are not exclusively associated with permanently silen t chromosomal domains. Our data support a view that members of this fa mily also associate with repressed states of euchromatin.