The evolution of DNA sequences in Escherichia coli

Area of Science:

• Molecular Biology
• Genetics
• Microbiology

Background:

• Transposable elements are mobile genetic sequences that can change their position within a genome.
• Plasmids are extrachromosomal DNA molecules that replicate independently of the bacterial chromosome.
• Horizontal gene transfer (HGT) is crucial for bacterial evolution and adaptation.

Purpose of the Study:

• To investigate the evolutionary origins of transposable elements.
• To explore the role of transposable elements in plasmid evolution and horizontal transmission.
• To examine the distribution and prevalence of insertion sequences (IS) in bacterial isolates.

Main Methods:

• Analysis of transposable elements (Tn3, gamma delta) known to be plasmid-associated.
• Distribution analysis of insertion sequences IS4 and IS5 in 67 natural isolates of Escherichia coli.
• Correlation analysis of copy numbers for different insertion sequences within strains.

Main Results:

• Transposable elements Tn3 and gamma delta are predominantly found in plasmids.
• Insertion sequences IS4 and IS5 were each present in approximately one-third of the studied E. coli strains.
• A negative correlation was observed between the copy numbers of IS4 and IS5 in strains containing both elements.
• Approximately half of the strains with a chromosomal copy of IS5 also harbored plasmid-located copies.

Conclusions:

• Transposable elements likely originated in plasmids, aiding in the horizontal transmission of non-conjugational plasmids.
• Insertion sequences show a widespread but variable distribution in natural E. coli populations.
• The findings support a role for transposable elements in plasmid biology and bacterial HGT.