Phosphorylation of linker histone is associated with transcriptional activation in a normally silent nucleus

Abstract

Previous studies have suggested that micronuclear linker histones are phosphorylated by cAMP-dependent protein kinase (PKA) in Tetrahymena (Sweet, M.T., and C.D. Allis. 1993. Chromosoma. 102: 637-647). In this study, we report that a rapid and dramatic phosphorylation of the micronuclear linker histone, δ, occurs early in the sexual pathway, conjugation. Phosphorylated isoforms of δ are detected as early as 30 min after mixing cells of different mating types; blocking pair formation abolishes this induction completely. Phosphorylation of δ is stimulated by the addition of N6- benzoyladenosine 3':5' cyclic monophosphate to starved (nonmating) cells, suggesting that a PKA/cAMP signal transduction pathway is involved. Maximal phosphorylation of δ is observed during meiotic prophase, a period when micronuclei become transcriptionally active. In situ staining, using phosphodelta-specific antibodies combined with [3H]uridine autoradiography, shows that decondensed micronuclear chromatin undergoing active transcription is enriched in phosphorylated δ isoforms. In contrast, condensed inactive micronuclear chromatin is enriched in dephosphorylated δ. These results strongly suggest that phosphorylation of linker histone plays an important and previously unsuspected role in establishing transcriptional competence in micronuclei.