A temperature-sensitive cell cycle arrest mutation affecting H1 phosphorylation and nuclear localization of a small heat shock protein in Tetrahymena thermophila.

This report describes a temperature-sensitive Tetrahymena thermophila cell cycle arrest mutant that is also deficient in its heat shock response. Mutants incubated at 41 degrees C undergo rapid dephosphorylation of macronuclear histone H1, in contrast to wild-type cells which hyperphosphorylate H1 under the same conditions. Dephosphorylation is specific to H1 and is associated with a threefold decrease in the level of H1 kinase activity in macronuclei isolated from heat-shocked mutants. A small nuclear heat shock protein, sp29c, is synthesized and phosphorylated normally in the mutant cells but fails to accumulate in macronuclei. Nuclear transport of other heat shock proteins is unaffected. Mutant cells die slowly at 41 degrees C, a temperature at which wild-type cells resume normal growth after a brief lag. Wild-type cells acquire thermotolerance (competence to survive a 3-h heat shock at 43 degrees C) after a 1-h treatment at 41 degrees C, but mutant cells cannot become thermotolerant and die after the same treatment. The mutation is named chp 1 (cell cycle, heat shock, and phosphorylation defect).