Volume 6, Issue 10 pp. 1395-1411
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Identification, genetic analysis and DNA sequence of a 7.8-kb virulence region of the Salmonella typhimurium virulence plasmid

Paul A. Gulig

Corresponding Author

Paul A. Gulig

Department of Immunology and Medical Microbiology, University of Florida College of Medicine, Gainesville, Florida 32610-0266, USA.

*For correspondence. Tel, (904) 392 0050; Fax (904) 392 3133.Search for more papers by this author
Allison L. Caldwell

Allison L. Caldwell

Department of Immunology and Medical Microbiology, University of Florida College of Medicine, Gainesville, Florida 32610-0266, USA.

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Vince A. Chiodo

Vince A. Chiodo

Department of Immunology and Medical Microbiology, University of Florida College of Medicine, Gainesville, Florida 32610-0266, USA.

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First published: May 1992
Citations: 70

Summary

The 90-kilobase (kb) virulence plasmid of Salmonella typhimurium is responsible for invasion from the intestines to mesenteric lymph nodes and spleens of orally inoculated mice. We used Tn5 and aminoglycoside phosphotransferase (aph) gene insertion mutagenesis and deletion mutagenesis of a previously identified 14-kb virulence region to reduce this virulence region to 7.8kb. The 7.8-kb virulence region subcloned into a low copy-number vector conferred a wild-type level of splenic infection to virulence plasmid-cured S. typhimurium and conferred essentially a wild-type oral LD50. Insertion mutagenesis identified five loci essential for virulence, and DNA sequence analysis of the virulence region identified six open reading frames. Expected protein products were identified from four of the six genes, with three of the proteins identified as doublet bands in Escherichia coli minicells. Three of the five mutated genes were able to be complemented by clones containing only the corresponding wild-type gene. Only one of the five deduced amino acid sequences, that of the positive regulatory element, SpvR, possessed significant homology to other proteins. The codon usage for the virulence genes showed no codon bias, which is consistent with the low levels of expression observed for the corresponding proteins. Consensus promoters for several different sigma factors were identified upstream of several of the genes, whereas only consensus Rho-dependent termination sequences were observed between certain of the genes. The operon structure of this virulence region therefore appears to be complex. The construction of the cloned 7.8-kb virulence region and the determination of the DNA sequence will aid in the further genetic analysis of the five plasmid-encoded virulence genes of S. typhimurium.

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