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Journal of Bacteriology, August 1999, p. 4949-4954, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

InvF Is Required for Expression of Genes Encoding Proteins Secreted by the SPI1 Type III Secretion Apparatus in Salmonella typhimurium

K. Heran Darwin1,2 and Virginia L. Miller2,3,*

Department of Microbiology and Molecular Genetics, University of California, Los Angeles, Los Angeles, California 90095,1 and Departments of Molecular Microbiology2 and Pediatrics,3 Washington University School of Medicine, St. Louis, Missouri 63110

Received 17 March 1999/Accepted 10 June 1999

The expression of genes encoding proteins secreted by the SPI1 (Salmonella pathogenicity island) type III secretion apparatus is known to require the transcriptional activators SirA and HilA. However, neither SirA nor HilA is believed to directly activate the promoters of these genes. invF, the first gene of the inv-spa gene cluster, is predicted to encode an AraC-type transcriptional activator and is required for invasion into cultured epithelial cells. However, the genes which are regulated by InvF have not been identified. In this work, an in-frame deletion in invF was constructed and tested for the expression of Phi (sigD-lacZYA), sipC::Tn5lacZY, and a plasmid-encoded Phi (sicA-lacZYA). SigD (Salmonella invasion gene) is a secreted protein required for the efficient invasion of Salmonella typhimurium into cultured eucaryotic cells. sicA (Salmonella invasion chaperone) is the first gene of a putative operon encoding the Sip/Ssp (Salmonella invasion/Salmonella secreted proteins) invasion proteins secreted by the SPI1 type III export apparatus. invF was required for the expression of the sigD, sicA, and sipC fusions. This is the first demonstration that there is a functional promoter in the intergenic sequence between spaS and sicA. In addition, several proteins were either absent from or found in reduced amounts in the culture supernatants of the invF mutant. Therefore, invF is required for the optimal expression of several genes encoding SPI1-secreted proteins. Genetic evidence is also presented suggesting there is HilA-dependent readthrough transcription from the invF promoter at least through sipC.

* Corresponding author. Mailing address: Department of Molecular Microbiology, Washington University School of Medicine, 660 South Euclid, Campus Box 8230, St. Louis, MO 63110. Phone: (314) 747-2132. Fax: (314) 747-2135. E-mail: virginia{at}borcim.wustl.edu.

Journal of Bacteriology, August 1999, p. 4949-4954, Vol. 181, No. 16
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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