Chromosomal replication incompatibility in Dam methyltransferase deficient Escherichia coli cells

Area of Science:

• Microbiology
• Molecular Biology
• Genetics

Background:

• Dam methyltransferase is crucial for DNA replication regulation in E. coli.
• Minichromosomes typically exist as extrachromosomal elements in E. coli.
• oriC is the essential origin of replication on the bacterial chromosome.

Purpose of the Study:

• To investigate the behavior of minichromosomes in Dam- deficient E. coli.
• To identify the mechanism behind minichromosome incompatibility with the host chromosome.
• To explore the role of oriC sequence in this phenomenon.

Main Methods:

• Construction of Dam methyltransferase deficient E. coli strains harboring minichromosomes.
• Analysis of plasmid DNA presence and integration.
• Utilizing oriC-pBR322 chimeric plasmids and deletion derivatives.
• Introduction of the seqA2 mutation to assess its effect on incompatibility.

Main Results:

• Free plasmid DNA was undetectable; minichromosomes integrated into the host oriC region.
• An intact and functional oriC sequence was identified as the incompatibility determinant.
• The seqA2 mutation increased cellular oriC copy number, resolving incompatibility.
• DNA replication initiation occurs at a fixed cell mass, regardless of chromosomal origin number.

Conclusions:

• Dam deficiency leads to minichromosome integration at the host oriC due to replication initiation cascade absence.
• Functional oriC is essential for plasmid-chromosome incompatibility.
• The seqA2 mutation can overcome oriC-mediated incompatibility by increasing origin copy number.
• Bacterial DNA replication initiation is cell-mass dependent, not origin-copy-number dependent.