Figure 2.

H1 reduces the number of replication forks in sperm nuclei but not the rate of fork movement. (A) Xenopus sperm chromatin was incubated for 90 min at 1 ng DNA/μl egg extract supplemented with Ara-CTP, [α-³²P]dATP, and histone H1 (H1) or without H1 (Control). Stalled replication forks were then released by addition of dCTP to a final concentration of 1 mM, and samples were collected at 2-min intervals for up to 8 min. The purified ³²P-labeled replication products were resolved on a 0.8% alkaline agarose gel that was subjected to autoradiography. The positions of DNA molecular mass markers are indicated. (B) The relative amount of radioactivity in the center of each lane in panel A was determined by densitometry, and these values were plotted as a function of molecular size in kilobases (kb). Data from samples collected immediately after dCTP addition to extract (0′), and after a 4-min (4′) and 8-min (8′) release are shown. The amplitudes of the control samples on the graph were reduced to facilitate comparison with the H1 samples.