FIG. 2.

(A) Histone H4 site II and H1⁰ H4 box are functionally equivalent in proliferating cells. The H4 box from the Xenopus histone H1⁰ gene promoter was replaced by the H4 site II sequence from a human histone H4 promoter (construct 2, H4H4). Conserved TCA and GT motifs in the H1⁰ H4 box were replaced by GTC and AA, respectively (TCA^m and GT^m constructs). Construct 1 is the wild-type Xenopus histone H1⁰ promoter. One microgram of each plasmid was used to transfect Cl6 cells; 48 h after transfection, CAT activity was measured. Plasmid uptake was controlled by Southern blot analysis of DNA extracted from transfected cells and hybridization with a ³²P-labeled plasmid probe. The measured CAT activity was then normalized with respect to the plasmid uptake. Data are presented as percentage of the activity detected from the wild-type promoter, and bars indicate standard deviations of two independent experiments. (B) Different transcriptional activities of histone H4 and H1⁰ genes during cell cycle exit in differentiating cells. B16 cells were induced to differentiate with 5 mM butyrate, and cells were taken at indicated times. A Northern blot containing these RNAs was probed successively with ³²P-labeled H1⁰ and H4 probes. The ethidium bromide-stained gel before the transfer of RNAs onto the membrane is also shown. (C) Early cell cycle exit after the induced differentiation of B16 cells. B16 cells were pulse-labeled with [³H]thymidine at the indicated times after the induced differentiation and analyzed; 100% represents [³H]thymidine incorporation in noninduced cells (means of three independent counts); bars indicate standard deviations.