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Journal of Bacteriology, February 1999, p. 1301-1308, Vol. 181, No. 4
Department of Organismic and Evolutionary
Biology, Harvard University, Cambridge, Massachusetts 02138
Received 26 May 1998/Accepted 1 December 1998
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.
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Analysis of the Type 1 Pilin Gene Cluster
fim in Salmonella: Its Distinct Evolutionary
Histories in the 5' and 3' Regions
*
Corresponding author. Mailing address: Department of
Organismic and Evolutionary Biology, Harvard University, Cambridge, MA 02138. Phone: (617) 496-3917. Fax: (617) 496-5854. E-mail:
dhartl{at}oeb.harvard.edu.
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