Effects of toxin B treatment on histone H3 Ser10 phosphorylation and cyclin B expression. (A) Immunofluorescence for histone H3 Ser10 phosphorylation and cyclin B in control and toxin B–treated HeLa cells. Toxin B was added to the culture 8 h after the second release, and cells were fixed at 10 h and stained for histone H3 Ser10 phosphorylation (red), cyclin B (green), and DNA (blue). Typical results of three independent experiments are shown. Bar, 10 μm. (B) Immunoblot analysis for histone H3 Ser10 phosphorylation and cyclin B. HeLa cells were collected every 2 h from 0 to 12 h after the second release, and lysates were prepared and subjected to immunoblot with antibodies to phosphoSer10-hisotne H3, cyclin B, and β-tubulin. Note the extensive phosphoSer10-histone H3 band in lysates of control cells at 10 and 12 h, which is absent in the corresponding lysates of toxin B–treated cells.