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Journal of Bacteriology, September 1998, p. 4452-4459, Vol. 180, No. 17
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Biochemistry and Regulation of a Novel Escherichia coli K-12 Porin Protein, OmpG, Which Produces Unusually Large Channels

Daniel A. Fajardo,1 Joyce Cheung,2 Chikako Ito,3 Etsuko Sugawara,3 Hiroshi Nikaido,3 and Rajeev Misra1,2,*

Department of Microbiology1 and Molecular and Cellular Biology Program,2 Arizona State University, Tempe, Arizona 85287-2701, and Department of Molecular and Cell Biology, University of California, Berkeley, California 94720-32063

Received 10 April 1998/Accepted 26 June 1998

A novel porin, OmpG, is produced in response to a chromosomal mutation termed cog-192. Molecular characterization of cog-192 revealed that it is a large chromosomal deletion extending from the 3' end of pspA through to the 5' end of an open reading frame located immediately upstream of ompG. As a result of this 13.1-kb deletion, the expression of ompG was placed under the control of the pspA promoter. Characterization of OmpG revealed that it is quite different from other porins. Proteoliposome swelling assays showed that OmpG channels were much larger than those of the OmpF and OmpC porins, with an estimated limited diameter of about 2 nm. The channel lacked any obvious solute specificity. The folding model of OmpG suggests that it is the first 16-stranded beta -barrel porin that lacks the large external loop, L3, which constricts the channels of other nonspecific and specific porins. Consistent with the folding model, circular dichroism showed that OmpG contains largely a beta -sheet structure. In contrast to other Escherichia coli porins, there is no evidence that OmpG exists as stable oligomers. Although ompG DNA was present in all E. coli strains examined so far, its expression under laboratory conditions was seen only due to rare chromosomal mutations. Curiously, OmpG was constitutively expressed, albeit at low levels, in Salmonella, Shigella, and Pseudomonas species.

* Corresponding author. Mailing address: Department of Microbiology, Arizona State University, Tempe, AZ 85287-2701. Phone: (602) 965-3320. Fax: (602) 965-0098. E-mail: rajeev.misra{at}asu.edu.

Journal of Bacteriology, September 1998, p. 4452-4459, Vol. 180, No. 17
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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