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Journal of Bacteriology, April 1999, p. 2132-2141, Vol. 181, No. 7
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.

The Major Phase-Variable Outer Membrane Protein of Escherichia coli Structurally Resembles the Immunoglobulin A1 Protease Class of Exported Protein and Is Regulated by a Novel Mechanism Involving Dam and OxyR

Ian R. Henderson* and Peter Owen

Department of Microbiology, Moyne Institute of Preventive Medicine, Trinity College Dublin, Dublin 2, Ireland

Received 14 October 1998/Accepted 24 January 1999

Here we report the characterization of an Escherichia coli gene (agn43) which encodes the principal phase-variable outer membrane protein termed antigen 43 (Ag43). The agn43 gene encodes a precursor protein of 107 kDa containing a 52-amino-acid signal sequence. Posttranslational processing generates an alpha 43 subunit (predicted Mr of 49,789) and a C-terminal domain (beta 43) with features typical of a bacterial integral outer membrane protein (predicted Mr of 51,642). Secondary structure analysis predicts that beta 43 exists as an 18-stranded beta  barrel and that Ag43 shows structural organization closely resembling that of immunoglobulin A1 protease type of exoprotein produced by pathogenic Neisseria and Haemophilus spp. The correct processing of the polyprotein to alpha 43 and beta 43 in OmpT, OmpP, and DegP protease-deficient E. coli strains points to an autocatalytic cleavage mechanism, a hypothesis supported by the occurrence of an aspartyl protease active site within alpha 43. Ag43, a species-specific antigen, possesses two RGD motifs of the type implicated in binding to human integrins. The mechanism of reversible phase variation was studied by immunochemical analysis of a panel of well-defined regulatory mutants and by analysis of DNA sequences upstream of agn43. Evidence strongly suggests that phase variation is regulated by both deoxyadenosine methylase (Dam) and by OxyR. Thus, oxyR mutants are locked on for Ag43 expression, whereas dam mutants are locked off for Ag43 expression. We propose a novel mechanism for the regulation of phase switching in which OxyR competes with Dam for unmethylated GATC sites in the regulatory region of the agn43 gene.

* Corresponding author. Present address: Center for Vaccine Development, 685 W. Baltimore St., Baltimore, MD 21201. Phone: (410) 706-7376. Fax: (410) 706-6205. E-mail: ihenders{at}umaryland.edu.

Journal of Bacteriology, April 1999, p. 2132-2141, Vol. 181, No. 7
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.


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