In vitro replication of plasmids containing human c-myc DNA

Area of Science:

• Molecular Biology
• Genetics
• Cell Biology

Background:

• A replication initiation zone was previously mapped to the 5' flanking DNA of the human c-myc gene in cell cultures.
• Understanding DNA replication origins is crucial for comprehending genome stability and cell proliferation.

Purpose of the Study:

• To investigate the in vitro replication of a plasmid containing the c-myc initiation zone.
• To characterize the mechanism and location of replication initiation within this specific DNA region.

Main Methods:

• In vitro replication assays using a plasmid (pNeo.Myc-2.4) containing the c-myc initiation zone.
• DpnI-resistance assay to detect newly synthesized DNA.
• Analysis of Okazaki fragments and DNA strand maturation.
• Two-dimensional gel electrophoresis and electron microscopy to visualize replication intermediates.
• Replication time course experiments.

Main Results:

• The plasmid containing the c-myc initiation zone replicated semiconservatively in vitro, producing supercoiled and relaxed monomers and replicative intermediates.
• Newly incorporated nucleotides ([32P]dCMP) were found in Okazaki fragments and matured into full-length DNA, distinguishing it from control plasmids.
• Replication initiation showed a preference for a specific restriction fragment within the c-myc origin zone.
• Two-dimensional electrophoresis and electron microscopy confirmed replication bubbles centered within the initiation zone, approximately 1.4 kb upstream of c-myc promoter P1.
• Okazaki fragment analysis revealed a switch in template preference asymmetry within the initiation zone.

Conclusions:

• Bidirectional, semiconservative DNA replication can preferentially originate in vitro within the 5' flanking DNA of the human c-myc gene.
• Replicative intermediates can be distinguished from repair-type processes.
• The study provides detailed mechanistic insights into DNA replication initiation at the c-myc locus.