Histone H1 is highly phosphorylated in transcriptionally active, amitotic macronuclei of Tetrahymena during vegetative growth. However, the level of H1 phosphorylation changes dramatically in response to different physiological conditions. H1 is hyperphosphorylated in response to heat shock and during prezygotic stages of conjugation. Conversely, H1 is largely dephosphorylated during prolonged starvation and during elimination of parental macronuclei during conjugation. Mapping of phosphorylation sites within H1 indicates that phosphorylation occurs at multiple sites in the amino-terminal portion of the molecule, predominantly at threonine residues. Two of these sites have been identified by compositional analyses and microsequencing of tryptic peptides. Interestingly, two major sites contain the sequence Thr-Pro-Val-Lys similar to that contained in the sites recognized by growth-associated histone kinase in other organisms. No new sites are detected during the hyperphosphorylation of H1 which occurs during heat shock or in early stages of conjugation, and no sites are preferentially dephosphorylated during starvation or later stages of conjugation. Therefore, changes in the overall level of H1 phosphorylation, as opposed to phosphorylation or dephosphorylation at particular sites, appear to be important in the regulation of chromatin structure under these physiological conditions. Further, since no cell division or DNA replication occurs under these conditions, changes in the level of H1 phosphorylation are best correlated to changes in gene expression during heat shock, starvation, and conjugation. We suggest that, at least in Tetrahymena, H1 hyperphosphorylation is used as a rapid and transient mechanism for the cessation of transcription under conditions of cellular stress.

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