The H1 phosphorylation state regulates expression of CDC2 and other genes in response to starvation in Tetrahymena thermophila

In Tetrahymena thermophila, highly phosphorylated histone HI of growing cells becomes partially dephosphorylated when cells are starved in preparation for conjugation. To determine the effects of HI phosphorylation on gene expression, PCR-based subtractive hybridization was used to clone cDNAs that were differentially expressed during starvation in two otherwise-isogenic strains differing only in their His. HI in A5 mutant cells lacked phosphorylation, and H1 in E5 cells mimicked constitutive HI phosphorylation. Sequences enriched in A5 cells included genes encoding proteases. Sequences enriched in E5 cells included genes encoding cdc2 kinase and a Ser/Thr kinase. These results indicate that HI phosphorylation plays an important role in regulating the pattern of gene expression during the starvation response and that its role in transcription regulation can be either positive or negative. Treatment of starved cells with a phosphatase inhibitor caused CDC2 gene overexpression. Expression of the E5 version of HI in starved cells containing endogenous, wild-type H1 caused the wild-type H1 to remain highly phosphorylated. These results argue that Cdc2p is the kinase that phosphorylates Tetrahymena H1, establish a positive feedback mechanism between H1 phosphorylation and CDC2 expression, and indicate that CDC2 gene expression is regulated by an H1 phosphatase.