Changes in chromatin and the phosphorylation of nuclear proteins during heat shock of Achlya ambisexualis

Heat shock led to marked changes in the apparent levels of phosphorylation of nuclear proteins in the fungus Achlya ambisexualis. We characterized these heat shock-induced changes in nuclear proteins on two types of two-dimensional polyacrylamide gel systems. We report here that one of two Achlya H3 histones (H3.1) and also the oomycete histone alpha appear to be highly phosphorylated with heat shock. Additional changes observed in acid-soluble nuclear proteins included an apparent increase in the 32P labeling of a 43,000-molecular-weight protein and the dephosphorylation of a major group of Achlya phosphoproteins in the 30,000-to-32,000-molecular-weight range. The changes in protein phosphorylation were accompanied by striking changes in the morphology of Achlya nuclei. Nuclei in the heat-shocked cells, but not in control cells, exhibited marked chromatin condensation and contained bundles of filaments which were approximately 4 nm in diameter. Concomitantly, the bulk of chromatin from heat-shocked nuclei showed a decreased sensitivity to digestion with the enzyme DNase I relative to chromatin from control cells.