A Nonessential HP1-Like Protein Affects Starvation-Induced Assembly of Condensed Chromatin and Gene Expression in Macronuclei of Tetrahymena thermophila

Abstract

Heterochromatin represents a specialized chromatin environment vital to both the repression and expression of certain eukaryotic genes. One of the best-studied heterochromatin-associated proteins is Drosophila HP1. In this report, we have disrupted all somatic copies of the Tetrahymena HHP1 gene, which encodes an HP1-like protein, Hhp1p, in macronuclei (H. Huang, E. A. Wiley, R. C. Lending, and C. D. Allis, Proc. Natl. Acad. Sci. USA 95:13624–13629, 1998). Unlike the Drosophila HP1 gene,HHP1 is not essential in Tetrahymena spp., and during vegetative growth no clear phenotype is observed in cells lacking Hhp1p (ΔHHP1). However, during a shift to nongrowth conditions, the survival rate of ΔHHP1 cells is reduced compared to that of wild-type cells. Upon starvation, Hhp1p becomes hyperphosphorylated concomitant with a reduction in macronuclear volume and an increase in the size of electron-dense chromatin bodies; neither of these morphological changes occurs in the absence of Hhp1p. Activation of two starvation-induced genes (ngoA and CyP) is significantly reduced in ΔHHP1 cells while, in contrast, the expression of several growth-related or constitutively expressed genes is comparable to that in wild-type cells. These results suggest that Hhp1p functions in the establishment and/or maintenance of a specialized condensed chromatin environment that facilitates the expression of certain genes linked to a starvation-induced response.

ACKNOWLEDGMENTS

We are grateful to Jody Bowen for her technical assistance in obtaining the ΔHHP1 transformants and for sharing many reagents, including the gene disruption cassette p4T2-1,HHT1, HHT3, ngoA, and CyPprobes. All ultrastructural analyses were carried out in the electron microscopy facility of the Biology Department at Syracuse University.

This research was supported by a grant from the National Institutes of Health to C.D.A. (GM40922).