FIG. 4.

The enzymatic activity of Hbo1 during the cell cycle. (A) A549 cells were synchronized by a double-thymidine block and release, and whole-cell extracts were prepared at the indicated time points. The whole lysates were probed with antiactin (Actin) as a loading control and anti-Hbo1 antibody (Hbo1). Anti-Hbo1 immunoprecipitates were assayed for HAT activity by the transfer of [³H]acetate to purified chicken histones (Fluorogram). The input histone levels were monitored by Coomassie staining (CBB Stain). (B) The cell cycle synchrony of the culture used for the data shown in panel A was followed by flow cytometry. At the times indicated, cells were sampled and stained with propidium iodide. Histograms of DNA fluorescence are shown. The histogram of the starting asynchronous culture is also shown (async). (C) The cell cycle progression of the culture was also followed by assaying the Cdc2-cyclin B1 kinase activity of cyclin-B1 immunoprecipitates using histone H1 as a substrate.