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Journal of Bacteriology, December 2002, p. 6976-6986, Vol. 184, No. 24
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.24.6976-6986.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

The Escherichia coli gabDTPC Operon: Specific {gamma}-Aminobutyrate Catabolism and Nonspecific Induction

Barbara L. Schneider, Stephen Ruback,{dagger} Alexandros K. Kiupakis, Hillary Kasbarian, Christine Pybus, and Larry Reitzer*

Department of Molecular and Cell Biology, The University of Texas at Dallas, Richardson, Texas 75083-0688

Received 5 July 2002/ Accepted 19 September 2002

Nitrogen limitation induces the nitrogen-regulated (Ntr) response, which includes proteins that assimilate ammonia and scavenge nitrogen. Nitrogen limitation also induces catabolic pathways that degrade four metabolically related compounds: putrescine, arginine, ornithine, and {gamma}-aminobutyrate (GABA). We analyzed the structure, function, and regulation of the gab operon, whose products degrade GABA, a proposed intermediate in putrescine catabolism. We showed that the gabDTPC gene cluster constitutes an operon based partially on coregulation of GabT and GabD activities and the polarity of an insertion in gabT on gabC. A {Delta}gabDT mutant grew normally on all of the nitrogen sources tested except GABA. The unexpected growth with putrescine resulted from specific induction of gab-independent enzymes. Nac was required for gab transcription in vivo and in vitro. Ntr induction did not require GABA, but various nitrogen sources did not induce enzyme activity equally. A gabC (formerly ygaE) mutant grew faster with GABA and had elevated levels of gab operon products, which suggests that GabC is a repressor. GabC is proposed to reduce nitrogen source-specific modulation of expression. Unlike a wild-type strain, a gabC mutant utilized GABA as a carbon source and such growth required {sigma}S. Previous studies showing {sigma}S-dependent gab expression in stationary phase involved gabC mutants, which suggests that such expression does not occur in wild-type strains. The seemingly narrow catabolic function of the gab operon is contrasted with the nonspecific (nitrogen source-independent) induction. We propose that the gab operon and the Ntr response itself contribute to putrescine and polyamine homeostasis.

* Corresponding author. Mailing address: Department of Molecular and Cell Biology, Mail Station FO 3.1, The University of Texas at Dallas, P.O. Box 830688, Richardson, TX 75083-0688. Phone: (972) 883-2502. Fax: (972) 883-2409. E-mail: reitzer{at}utdallas.edu.

{dagger} Present address: Cancer Immunobiology Center, University of Texas Southwestern Medical School, Dallas, TX 75235-8576.

Journal of Bacteriology, December 2002, p. 6976-6986, Vol. 184, No. 24
0021-9193/02/$04.00+0     DOI: 10.1128/JB.184.24.6976-6986.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.



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