Change of plasmid DNA structure, hypermethylation, and Lon-proteolysis as steps in a replicative cascade

Area of Science:

• Molecular Biology
• Genetics
• Biochemistry

Background:

• Plasmid DNA maintenance is crucial for bacterial genetics.
• Understanding DNA helical density variations can reveal replication mechanisms.

Purpose of the Study:

• To define conditions for maintaining RepFIC plasmid DNA at a lowered helical density.
• To investigate the role of methylation and host factors in this DNA state.

Main Methods:

• Culturing RepFIC plasmid in specific bacterial hosts.
• Analyzing DNA helical density and methylation patterns.
• Investigating the effect of host mutations (dam and lon).

Main Results:

• Identified conditions for lowered helical density RepFIC plasmid DNA.
• This variant is present in exponentially growing cultures, suggesting it's a normal maintenance form.
• The DNA is methylated by dam-methyltransferase at recognized and novel sites.
• Methylation is not essential for the helical density change, as it occurs in dam hosts.
• Lowered helical density is stabilized in lon hosts.

Conclusions:

• Lowered helical density RepFIC plasmid DNA is a normal variant, potentially a replication precursor.
• Methylation by dam-methyltransferase is involved but not essential for the helical density change.
• Lon-protease likely plays a role in regulating this DNA state by degrading associated proteins.