FIG. 3.

Quantification of DNA content showed that after 24 h incubation in serum free medium synchronized BCBL-1 cells were arrested in G₀, as shown by the majority of cells having a 2N DNA content and no BrdU incorporation (A, left panel). However, 16 h after release from G₀ block, cells reach S phase of the cell cycle, as shown by an increase in the percentage of cells that have incorporated BrdU into newly synthesized DNA (A, right panel). Blockade at early S phase following HU treatment was verified by the same method (B). HU-treated cultures had significantly reduced numbers of cells progressing through S phase (6%; B, upper right quadrant of right panel) compared with untreated cultures (58%; upper right quadrant of left panel). Expression of the late lytic gene ORF 65 (determined by RT-PCR analysis) was greater in TPA-induced cultures of BCBL-1 cells that had been previously synchronized at early S phase of the cell cycle by incubation with HU (C).