The gab operon (gabDTPC) in Escherichia coli functions in the conversion of ␥-aminobutyrate to succinate. One component of gab operon regulation involves the RpoS sigma factor, which mediates activation at high cell density. Transposon mutagenesis was used to identify new genes that regulate gab operon expression in rich media. A Tn5tmp insertion in the hldD (formerly rfaD) gene increased gabT::lacZ expression 12-fold. The hldD gene product, an ADP-L-glycerol-D-mannoheptose-6-epimerase, catalyzes the conversion of ADP-D-glycerol-Dmannoheptose to ADP-L-glycerol-D-mannoheptose, a precursor for the synthesis of inner-core lipopolysaccharide (LPS). Defined mutations in hldE, required for heptose synthesis, and waaF, required for the addition of the second heptose to the inner core, also resulted in high-level gabT::lacZ expression. The hldD, hldE, and waaF mutants exhibited a mucoid colony phenotype due to production of a colanic acid capsule. However, in the hldD::cat background, the high-level expression of gabT::lacZ was independent of the regulatory components for colanic acid synthesis (rcsA, rcsB, and rcsC) and also independent of manC (cpsB), a structural gene for colanic acid synthesis. Activation of gabT::lacZ in the hldD::cat background was dependent on the RpoS sigma factor. The hldD::cat mutation resulted in a sixfold increase in the levels of a translational RpoS-LacZ fusion and had a marginal effect on a transcriptional fusion. This study reveals a stress-induced pathway, mediated by loss of the LPS inner core, that increases RpoS translation and gab operon expression in E. coli.The gabDTPC operon in Escherichia coli functions in the conversion of ␥-aminobutyrate (GABA) to succinate (3, 17). The gabD gene encodes a succinate:semialdehyde dehydrogenase that generates succinate from succinate-semialdehyde. The gabT gene encodes a succinate:semialdehyde aminotransferase that catalyzes the formation of succinate semialdehyde from GABA. The gabP gene encodes a GABA permease (3). In addition to the catabolism of ␥-aminobutyrate, the gab operon has been proposed to contribute to polyamine homeostasis during nitrogen-limited growth (25) and to maintain high internal glutamate concentrations under stress conditions (14).Regulation of the gab operon occurs at multiple levels. At least three promoters that transcribe the gab operon have been identified. The RpoS-encoded sigma factor S or 38 transcribes the gab operon, and recent studies indicate that two RpoS-dependent promoters are present upstream of the gab operon (2, 14, 23). In addition, a 70 -dependent promoter, together with the Nac regulatory protein, contributes to expression under nitrogen-limiting conditions (25,33). In nitrogen-rich environments, two accumulated extracellular signals, indole and a second unidentified signal, independently contribute to activation of the gab operon at high cell density in an RpoS-dependent manner (2, 31). Expression of the gab genes is also enhanced at high pH (28). Negative regulation of gab expression is mediated by ...