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Applied and Environmental Microbiology, May 2007, p. 2919-2930, Vol. 73, No. 9
0099-2240/07/$08.00+0     doi:10.1128/AEM.02664-06
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

The Alternative Sigma Factor ^B and the Virulence Gene Regulator PrfA Both Regulate Transcription of Listeria monocytogenes Internalins ^,

Department of Food Science, Cornell University, Ithaca, New York 14853

Received 14 November 2006/ Accepted 21 February 2007

Some Listeria monocytogenes internalins are recognized as contributing to invasion of mammalian tissue culture cells. While PrfA is well established as a positive regulator of L. monocytogenes virulence gene expression, the stress-responsive ^B has been recognized only recently as contributing to expression of virulence genes, including some that encode internalins. To measure the relative contributions of PrfA and ^B to internalin gene transcription, we used reverse transcription-PCR to quantify transcript levels for eight internalin genes (inlA, inlB, inlC, inlC2, inlD, inlE, inlF, and inlG) in L. monocytogenes 10403S and in isogenic prfA, sigB, and sigB prfA strains. Strains were grown under defined conditions to produce (i) active PrfA, (ii) active ^B and active PrfA, (iii) inactive PrfA, and (iv) active ^B and inactive PrfA. Under the conditions tested, ^B and PrfA contributed differentially to the expression of the various internalins such that (i) both ^B and PrfA contributed to inlA and inlB transcription, (ii) only PrfA contributed to inlC transcription, (iii) only ^B contributed to inlC2 and inlD transcription, and (iv) neither ^B nor PrfA contributed to inlF and inlG transcription. inlE transcript levels were undetectable. The important role for ^B in regulating expression of L. monocytogenes internalins suggests that exposure of this organism to environmental stress conditions, such as those encountered in the gastrointestinal tract, may activate internalin transcription. Interplay between ^B and PrfA also appears to be critical for regulating transcription of some virulence genes, including inlA, inlB, and prfA.

Published ahead of print on 2 March 2007.

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