Dear Dr. McFadden
A very interesting hypothesis you are following up. I also prefer to
consider that insertion elements have been maintained in bacteria because
they may give the strains carrying them a selective advantage (e.g. by
providing genetic flexibility) rather than because they transpose at
frequencies sufficiently high to compensate for losses incurred through
elimination of certain clones.
As far as IS200 is concerned, it is not Salmonella-specific, but may have
been present in a strain ancestral to both E. coli and Salmonella, as
isoforms of the element are found in some isolates of E. coli. The
available evidence from sequencing (see M. Bisercic and H. Ochman,
Genetics (1993) 133:449-454) would be compatible with this hypothesis.
Furthermore, related elements may have spread to other organisms (see M.
Simonet & al. Infect.Immun. (1996) 64:375-379).
What intrigues me is the close association of (other) insertion elements
with genes involved in bacterial virulence.
Andre P. Burnens MD
Institute for Veterinary Bacteriology, University of Berne
Laenggass-Strasse 122, CH-3012 Berne, Switzerland
Tel. +41 31 631 24 85 Fax. +41 31 631 26 34