Affiliation:
1. Department of Organismic and Evolutionary Biology, Harvard University, Cambridge, Massachusetts 02138
Abstract
ABSTRACT
The type 1 pilin encoded by
fim
is present in both
Escherichia coli
and
Salmonella
natural isolates, but several lines of evidence indicate that similarities at the
fim
locus may be an example of independent acquisition rather than common ancestry. For example, the
fim
gene cluster is found at different chromosomal locations and with distinct gene orders in these closely related species. In this work we examined the
fim
gene cluster of
Salmonella
, the genes of which show high nucleotide sequence divergence from their
E. coli
counterparts, as well as a different G+C content and codon usage. DNA hybridization analysis revealed that, among the salmonellae, the
fim
gene cluster is present in all isolates of
S. enterica
but is absent from
S. bongori
. Molecular phylogenetic analyses of the
fimA
and
fimI
genes yield an estimate of phylogeny that is in satisfactory congruence with housekeeping and other virulence genes examined in this species. In contrast, phylogenetic analyses of the
fimZ
,
fimY
, and
fimW
genes indicate that horizontal transfer of this region has occurred more than once. There is also size variation in the
fimZ
,
fimY
, and
fimW
intergenic regions in the 3′ region, and these genes are absent in isolate S2983 of subspecies IIIa. Interestingly, the G+C contents of the
fimZ
,
fimY
, and
fimW
genes are less than 46%, which is considerably lower than those of the other six genes of the
fim
cluster. This study demonstrates that horizontal transmission of all or part of the same gene cluster can occur repeatedly, with the result that different regions of a single gene cluster may have different evolutionary histories.
Publisher
American Society for Microbiology
Subject
Molecular Biology,Microbiology
Cited by
31 articles.
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