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Journal of Bacteriology, April 2000, p. 2262-2268, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
Complex Function for SicA, a Salmonella
enterica Serovar Typhimurium Type III Secretion-Associated
Chaperone
Stephanie C.
Tucker
and
Jorge E.
Galán*
Section of Microbial Pathogenesis, Boyer
Center for Molecular Medicine, Yale School of Medicine, New Haven,
Connecticut 06536
Received 15 November 1999/Accepted 25 January 2000
Salmonella enterica encodes a type III secretion system
within a pathogenicity island located at centisome 63 that is essential for virulence. All type III secretion systems require the function of a
family of low-molecular-weight proteins that aid the secretion process
by acting as partitioning factors and/or secretion pilots. One such
protein is SicA, which is encoded immediately upstream of the type III
secreted proteins SipB and SipC. We found that the absence of SicA
results in the degradation of both SipB and SipC. Interestingly, in the
absence of SipC, SipB was not only stable but also secreted at
wild-type levels in a sicA mutant background, indicating
that SicA is not required for SipB secretion. We also found that SicA
is capable of binding both SipB and SipC. These results are consistent
with a SicA role as a partitioning factor for SipB and SipC, thereby
preventing their premature association and degradation. We also found
that introduction of a sicA null mutation results in the
lack of expression of SopE, another type III-secreted protein. Such an
effect was shown to be transcriptional. Introduction of a
loss-of-function sipC mutation into the sicA mutant background rescued sopE expression. These results
indicate that the effect of sicA on sopE
expression is indirect and most likely exerted through a regulatory
factor(s) partitioned by SicA from SipC. These studies therefore
describe a surprisingly complex function for the Salmonella
enterica type III secretion-associated chaperone SicA.
*
Corresponding author. Mailing address: Section of
Microbial Pathogenesis, Boyer Center for Molecular Medicine, Yale
School of Medicine, New Haven, CT 06536. Phone: (203) 737-2404. Fax: (203) 737-2630. E-mail: jorge.galan{at}yale.edu.

Present address: Department of Medicine, Albert Einstein College of
Medicine, Bronx, NY 10461-1602.
Journal of Bacteriology, April 2000, p. 2262-2268, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.
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