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 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 (DeltaHHP1). However, during a shift to nongrowth conditions, the survival rate of DeltaHHP1 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 DeltaHHP1 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.
Huang
H
H
Department of Biology, Syracuse University, Syracuse, New York 13244, USA.
Smothers
J F
JF
Wiley
E A
EA
Allis
C D
CD
D000818:Animals; D002455:Cell Division; D002467:Cell Nucleus; D002843:Chromatin / Q000235:genetics* / Q000648:ultrastructure; D000090266:Chromobox Protein Homolog 5; D002868:Chromosomal Proteins, Non-Histone / Q000235:genetics* / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D017353:Gene Deletion; D005786:Gene Expression Regulation / Q000235:genetics; D010766:Phosphorylation; D015800:Protozoan Proteins / Q000235:genetics*; D012333:RNA, Messenger / Q000378:metabolism; D016808:Tetrahymena thermophila / Q000235:genetics*; D014170:Transformation, Genetic
D002843:Chromatin; D002868:Chromosomal Proteins, Non-Histone; D015800:Protozoan Proteins; D012333:RNA, Messenger; D000090266:Chromobox Protein Homolog 5