Early initiation of bovine satellite I DNA replication

Synchronized bovine cells were used to study-the replication of satellite I DNA during the first 3 h of S phase. Following isopyknic ultracentrifugation, we compared the specific activities of pulselabelled satellite I and main band DNA. In addition, the specific activity of the Eco RI fragment of satellite I DNA was determined quantitatively and visualized qualitatively. All methods showed an increase in the specific activity of satellite I concurrent with that of main band DNA near the beginning of S phase, first detectable at 1−1 1 2 h into S phase. Thus, although most satellite DNAs replicated late in S phase [1], we report here that bovine satellite I initiates its replication early in S phase. The implications of this result for the control of DNA synthesis are discussed.