Figure 1.

H1 reduces the rate and extent of DNA replication but does not affect the length of S-phase in egg extract. Xenopus sperm chromatin was incubated for 90 min at 1 ng DNA/μl egg extract supplemented with 200 μM Ara-CTP, [α-³²P]dATP, and with histone H1 (H1) or without H1 (Control). Parallel experiments showed that >95% of both control and H1 nuclei initiated replication during this initial incubation period, as judged by the incorporation of biotin-dUTP into nascent DNA (our unpublished observations). Stalled replication forks were then released by addition of dCTP to a final concentration of 1 mM, and the extent of DNA synthesis was determined at 10-min intervals up to 80 min. DNA replication is expressed as a percentage of the control sample at 80 min. Shown are the mean values, along with the SEM, from three separate experiments in which three different extracts were used.