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Genetic variation in IncI1-ColIb plasmids

F J Ayala1, D E Krane, D L Hartl

  • 1Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138.

Journal of Molecular Evolution
|August 1, 1994
PubMed
Summary

Salmonella and E. coli share common colicin plasmids, indicating horizontal gene transfer. These plasmids, including the abortive infection (abi) gene, show remarkable stability over decades and suggest diversifying selection.

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Area of Science:

  • Microbiology
  • Genetics
  • Evolutionary Biology

Background:

  • Colicin plasmids are mobile genetic elements found in enteric bacteria.
  • IncI1-ColIb plasmids are a specific type of colicin plasmid.
  • Understanding plasmid evolution and transfer is crucial for tracking bacterial adaptation.

Purpose of the Study:

  • To investigate the genetic relatedness and evolutionary dynamics of IncI1-ColIb plasmids in Salmonella typhimurium.
  • To compare plasmid gene sequences with chromosomal genes to infer transfer and selection mechanisms.
  • To assess the stability of these plasmids over extended historical periods.

Main Methods:

  • Nucleotide sequencing of plasmid genes (cib, cir, abi) and the chromosomal gene (gnd) from historical and recent Salmonella typhimurium isolates.
  • Comparative sequence analysis to identify polymorphisms and evolutionary patterns.
  • Bioinformatic analysis to infer selection pressures and gene transfer events.

Main Results:

  • Plasmid genes in Salmonella were nearly identical to those in E. coli, suggesting shared plasmid pools.
  • Plasmid genes exhibited lower polymorphism than chromosomal genes, supporting horizontal transfer and rapid turnover.
  • No significant genetic differences were observed between plasmids from 1935-1941 and 1974 isolates, indicating long-term stability.
  • An excess of amino acid replacement polymorphisms in plasmid genes suggests diversifying selection.
  • The abi gene contained two single-nucleotide insertions, altering the predicted protein length.

Conclusions:

  • Salmonella and E. coli share IncI1-ColIb plasmids, highlighting interspecies plasmid mobility.
  • These plasmids are evolutionarily stable over decades, with evidence of horizontal transfer and selection.
  • Diversifying selection may drive the evolution of colicin-producing plasmid families.
  • The abi gene sequence variations warrant further investigation into their functional implications.

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