A nonessential HP1-like protein affects starvation-induced assembly of condensed chromatin and gene expression in macronuclei of Tetrahymena thermophila

Heterochromatin represents a specialized chromatin environment vital to bot h the repression and expression of certain eukaryotic genes. One of the bes t-studied heterochromatin-associated proteins is Drosophila HP1. In this re port, we have disrupted all somatic copies of the Tetrahymena HHP1 gene, wh ich encodes an HP1-like protein, Hhp1p, in macronuclei (H. Huang, E, A. Wil ey, R, C. Lending, and C. D, Allis, Proc. Natl. Acad. Sci. USA 95:13624-136 29, 1998). Unlike the Drosophila HP1 gene, HHP1 is not essential in Tetrahy mena spp., and during vegetative growth no clear phenotype is observed in c ells lacking Hhp1p (Delta HHP1). However, during a shift to nongrowth condi tions, the survival rate of Delta HHP1 cells is reduced compared to that of wild-type cells. Upon starvation, Hhp1p becomes hyperphosphorylated concom itant with a reduction in macronuclear volume and an increase in the size o f electron-dense chromatin bodies; neither of these morphological changes o ccurs in the absence of Hhp1p. Activation of two starvation-induced genes ( ngoA and CyP) is significantly reduced in Delta HHP1 cells while, in contra st, the expression of several growth-related or constitutively expressed ge nes is comparable to that in mild-type cells. These results suggest that Hh p1p functions in the establishment and/or maintenance of a specialized cond ensed chromatin environment that facilitates the expression of certain gene s linked to a starvation-induced response.