DNA replication in vitro starting with an intact phiX174 phage

Area of Science:

• Molecular Biology
• Virology
• Biochemistry

Background:

• Bacteriophage phiX174 is a well-studied model organism for DNA replication.
• Understanding phage DNA replication is crucial for developing antiviral strategies.
• Previous studies indicated the involvement of cellular factors in phage DNA synthesis.

Purpose of the Study:

• To investigate the mechanism of single-stranded DNA to replicative form conversion in intact phiX174 phage particles.
• To identify the cellular components essential for this early stage of phage DNA replication.
• To characterize the properties of the active cellular fraction involved in the conversion process.

Main Methods:

• Phage-sensitive cell lysis and preparation of cellular fractions.
• In vitro assay for phiX174 single-stranded DNA to replicative form conversion.
• Rifampicin resistance assay to assess the involvement of host transcription.
• Purification of active fractions using nonionic detergents and polypropylene film adsorption.
• Buoyant density centrifugation to characterize the purified fraction.

Main Results:

• Conversion of phiX174 ssDNA to RF form was achieved in cell lysates.
• The conversion process is resistant to rifampicin, indicating a DNA replication-dependent mechanism.
• A membrane fraction and a multienzyme replicative system are essential for conversion.
• Lipopolysaccharide receptor, while necessary, cannot substitute for the membrane fraction.
• Nonsedimentable detergent extracts effectively replaced the membrane fraction.
• Purified active fractions showed a 5-fold increase in activity over lipopolysaccharide and a 50-fold increase over protein.
• The purified fraction has a low buoyant density (1.03 g/cm3), suggesting high phospholipid or detergent content.

Conclusions:

• The conversion of phiX174 ssDNA to RF form is an active, host-cell-dependent process requiring specific membrane-associated factors and a multienzyme system.
• A detergent-extractable, membrane-associated component plays a critical role in initiating phage DNA replication.
• The purified active fraction represents a key cellular machinery involved in early phage DNA replication, potentially involving membrane lipids or detergent complexes.