Résumé
SignificanceIn various eukaryotes, heterochromatin is cytologically visible because multiple heterochromatic loci are assembled into higher order structures called heterochromatin bodies. Although heterochromatin bodies are dynamically assembled and disassembled during development, aging, and carcinogenesis, their role and the mechanism of formation remain to be clarified. In this report, by investigating the role of the heterochromatin body component Jub4p in the de novo heterochromatin body formation process in the ciliated protozoan Tetrahymena, we reveal that the phosphorylation and subsequent dephosphorylation of the Heterochromatin Protein 1-like protein Pdd1p are prerequisites for heterochromatin body formation and the following programmed DNA elimination. This study demonstrates the biochemical and biological importance of a temporally ordered phosphorylation–dephosphorylation cycle of a heterochromatin component in heterochromatin body formation.
Multiple heterochromatic loci are often clustered into a higher order nuclear architecture called a heterochromatin body in diverse eukaryotes. Although phosphorylation of Heterochromatin Protein 1 (HP1) family proteins regulates heterochromatin dynamics, its role in heterochromatin bodies remains unknown. We previously reported that dephosphorylation of the HP1-like protein Pdd1p is required for the formation of heterochromatin bodies during the process of programmed DNA elimination in the ciliated protozoan Tetrahymena. Here, we show that the heterochromatin body component Jub4p is required for Pdd1p phosphorylation, heterochromatin body formation, and DNA elimination. Moreover, our analyses of unphosphorylatable Pdd1p mutants demonstrate that Pdd1p phosphorylation is required for heterochromatin body formation and DNA elimination, whereas it is dispensable for local heterochromatin assembly. Therefore, both phosphorylation and the following dephosphorylation of Pdd1p are necessary to facilitate the formation of heterochromatin bodies. We suggest that Jub4p-mediated phosphorylation of Pdd1p creates a chromatin environment that is a prerequisite for subsequent heterochromatin body assembly and DNA elimination.