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Infection and Immunity, March 1999, p. 1331-1337, Vol. 67, No. 3
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

Hyperproduction of Alpha-Hemolysin in a sigB Mutant Is Associated with Elevated SarA Expression in Staphylococcus aureus

Ambrose L. Cheung,1,* Yueh-tyng Chien,1 and Arnold S. Bayer2,3,4

Laboratory of Bacterial Pathogenesis and Immunology, The Rockefeller University, New York, New York 100211; St. John's Cardiovascular Research Center2 and Division of Infectious Diseases,3 Harbor-UCLA Medical Center, Torrance, California 90509; and UCLA School of Medicine, Los Angeles, California 900244

Received 9 October 1998/Returned for modification 4 December 1998/Accepted 14 December 1998

To evaluate the role of SigB in modulating the expression of virulence determinants in Staphylococcus aureus, we constructed a sigB mutant of RN6390, a prototypic S. aureus strain. The mutation in the sigB gene was confirmed by the absence of the SigB protein in the mutant on an immunoblot as well as the failure of the mutant to activate sigma B-dependent promoters (e.g., the sarC promoter) of S. aureus. Phenotypic analysis indicated that both alpha-hemolysin level and fibrinogen-binding capacity were up-regulated in the mutant strain compared with the parental strain. The increase in fibrinogen-binding capacity correlated with enhanced expression of clumping factor and coagulase on immunoblots. The effect of the sigB mutation on the enhanced expression of the alpha-hemolysin gene (hla) was primarily transcriptional. Upon complementation with a plasmid containing the sigB gene, hla expression returned to near parental levels in the mutant. Detailed immunoblot analysis as well as a competitive enzyme-linked immunosorbent assay of the cell extract of the sigB mutant with anti-SarA monoclonal antibody 1D1 revealed that the expression of SarA was higher in the mutant than in the parental control. Despite an elevated SarA level, the transcription of RNAII and RNAIII of the agr locus remained unaltered in the sigB mutant. Because of a lack of perturbation in agr, we hypothesize that inactivation of sigB leads to increased expression of SarA which, in turn, modulates target genes via an agr-independent but SarA-dependent pathway.

* Corresponding author. Mailing address: Laboratory of Bacterial Pathogenesis and Immunology, The Rockefeller University, 1230 York Ave., New York, NY 10021. Phone: 212-327-8163. Fax: 212-327-7584. E-mail: cheunga{at}rockvax.rockefeller.edu.

Infection and Immunity, March 1999, p. 1331-1337, Vol. 67, No. 3
0019-9567/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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