DNA Replication Initiates Non-Randomly at Multiple Sites Near the c-myc Gene in HeLa Cells

The origin of replication of the c-myc gene in HeLa cells was previously identified at low resolution within 3.5 kb 5′ to the P1 promoter, based on replication fork polarity and the location of DNA nascent strands. To define the initiation events in the c-myc origin at higher resolution the template bias of nascent DNAs in a 12 kb c-myc domain has been analyzed by hybridization to strand specific probes. Strong switches in the asymmetry of nascent strand template preference confirm that replication initiates non-randomly at multiple sites within 2.4 kb 5′ to the c-myc P1 promoter, and at other sites over a region of 12 kb or more. The strongest template biases occur in the 2.4 kb region 5′ of the P1 promoter, shown earlier to contain sequences which allow the autonomous semiconservative replication of c-myc plasmids. An asymmetric pyrimidine heptanucleotide consensus sequence has been identified which occurs 12 times in the c-myc origin zone, and whose polarity exactly correlates with the polarity of nascent strand synthesis.