Running title: steA is regulated by DsbA, MgrB and PhoP in Salmonella 4 5 6

JB Accepts, published online ahead of print on 15 March 2013 2 ABSTRACT 18 SteA is a protein that can be translocated into host cells through the two virulence-related 19 type III secretion systems that are present in Salmonella enterica. We used the T-POP 20 system to carry out general screens for loci that exhibited activation or repression of a 21 steA::lacZ fusion. These screens identified the histidine kinase PhoQ and the response 22 regulator PhoP as positive regulators of steA. Transcription of this gene is sigma70-23 dependent and the promoter of steA contains a PhoP-binding site that mediates direct 24 regulation by PhoP. Our screens also detected MgrB (also known as YobG) as a negative 25 regulator of the expression of steA. Disruption of the gene encoding the periplasmic 26 disulfide oxidoreductase DsbA or addition of the reducing agent dithiothreitol increase 27 transcription of steA. The effects of MgrB and DsbA on steA are mediated by PhoP. These 28 results suggest that the cellular redox status is a factor contributing to regulation of steA 29 and, probably, other virulence genes regulated by the two-component system PhoQ/PhoP. 3