Heterochromatic deposition of centromeric histone H3-like proteins

摘要

Centromeres of most organisms are embedded within constitutive heterochromatin, the condensed regions of chromosomes that account for a large fraction of complex genomes. The functional significance of this centromere--heterochromatin relationship, if any, is unknown. One possibility is that heterochromatin provides a suitable environment for assembly of centromere components, such as special centromeric nucleosomes that contain distinctive histone H3-like proteins. We describe a Drosophila H3-like protein, Cid (for centromere identifier) that localizes exclusively to fly centromeres. When the cid upstream region drives expression of H3 and HZB histone--green fluorescent protein fusion genes in Drosophila cells. euchromatin-specific deposition results. Remark- ably, when the Cid upstream region drives expression of yeast, worm, and human centromeric histone--green fluorescent protein fusion proteins, localization is preferentially within Drosophila pericentric heterochromatin. Heterochromatin-specific localization aIso was seen for yeast and worm centromeric proteins constitu- tively expressed in human cells. Preferential localization to het- erochromatin in heterologous systems is unexpected if centro- mere-specific or site-specific factors determine H3-like protein IocaIization to centromeres. Rather. the heterochromatic state itself may help localize centromeric components.