High-molecular-weight linear multimer formation by single-stranded DNA plasmids in Escherichia coli

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• Plasmids are crucial genetic elements in bacteria, often replicating via rolling-circle or theta mechanisms.
• Homologous recombination and DNA repair pathways in Escherichia coli play significant roles in maintaining genomic integrity.
• High-molecular-weight plasmid multimers (HMW) represent unusual DNA structures with implications for genetic stability.

Purpose of the Study:

• To investigate the formation of high-molecular-weight plasmid multimers (HMW) in Escherichia coli.
• To determine the influence of foreign DNA inserts, plasmid replication mechanisms, and host recombination functions on HMW formation.
• To elucidate the structural characteristics and formation mechanism of HMW.

Main Methods:

• Insertion of foreign DNA segments into single-stranded DNA (ssDNA) rolling-circle plasmids and theta replicons in E. coli.
• Analysis of HMW formation in wild-type, recA, and lexA (Ind-) mutant strains.
• Enzymatic characterization of HMW structures and assessment of their impact on other plasmids in heteroplasmid strains.

Main Results:

• HMW formation was observed with ssDNA rolling-circle plasmids but not with theta replicons upon foreign DNA insertion.
• HMW formation was dependent on host homologous recombination functions, being absent in recA mutants but present in lexA (Ind-) mutants.
• Enzymatic studies defined HMW as linear double-stranded tandem head-to-tail plasmid repeats, distinct from HMW formed without RecBCD activity.

Conclusions:

• Foreign DNA inserts can induce HMW formation in rolling-circle plasmids, contingent on replication mode and host recombination.
• The foreign insert likely acts to shield the plasmid DNA from RecBCD exonuclease activity.
• This phenomenon provides a novel system for studying plasmid DNA stability and recombination in E. coli.