Cointegrate formation between homologous plasmids in Escherichia coli

Area of Science:

• Molecular Biology
• Microbiology
• Bacterial Genetics

Background:

• Conjugation is a key mechanism for bacterial genetic exchange.
• Plasmid replication and stability are crucial for maintaining genetic elements within bacteria.
• The polA amber mutant JG112 recipient strain prevents replication of certain plasmids, facilitating analysis of genetic transfer.

Purpose of the Study:

• To investigate the role of cloned DNA fragments from the R plasmid NR1 in promoting cointegrate formation with a ColE1 derivative (RSF2124).
• To determine the frequency of antibiotic resistance cotransfer during conjugation under different conditions.
• To analyze the molecular basis of genetic linkage and transposition between plasmids and the bacterial chromosome.

Main Methods:

• Bacterial conjugation experiments using Escherichia coli K-12 strains.
• Plasmid DNA isolation and analysis via gel electrophoresis.
• Cloning of specific EcoRI fragments (A and H) from NR1 into RSF2124.

Main Results:

• Cloning EcoRI fragment A of NR1 into RSF2124 resulted in 25-60% cotransfer of ampicillin and tetracycline resistance, forming cointegrates.
• Cloning EcoRI fragment H of NR1 into RSF2124 yielded about 4% cotransfer, with most transconjugants containing cointegrates.
• Without cloned fragments, less than 0.1% cotransfer occurred, with Tn3 transposition onto NR1 observed.

Conclusions:

• Homologous DNA sequences between plasmids significantly increase the formation of cointegrates within host cells.
• The size and location of cloned DNA fragments influence the efficiency of cointegrate formation and antibiotic resistance transfer.
• Cointegrate formation is a pre-conjugation event, suggesting an intrinsic cellular mechanism rather than mating-induced stimulation.