We have characterized two genes of the Escherichia coli K-12 gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway. The nucleotide sequence of gabT, coding for glutamate:succinic semialdehyde transaminase (EC 2.6.1.19), alternatively known as 4-aminobutyrate transaminase, was determined. The structural gene consists of 1,281 nucleotides specifying a protein of 426 amino acids with a molecular mass of 45.76 kDa. The protein shows significant homologies to the ornithine transaminases from Saccharomyces cerevisiae and from rat and human mitochondria. Three functionally and structurally important amino acid residues of the transaminase were identified by sequence comparison studies, and evolutionary relationships of the aminotransferases are discussed. The gabD gene, encoding succinic semialdehyde dehydrogenase (EC 1.2.1.16), was cloned and shown to be located adjacent to the 5' end of gabT. Expression studies with subfragments of the initially cloned DNA region revealed a maximal size of 1.7 kb for gabD. Both genes are cotranscribed from a promoter located upstream of gabD.The gab cluster of Escherichia coli specifies the synthesis of the enzymes of the 4-aminobutyrate (GABA) degradation pathway (8,9). The cluster, which is located at 57.6 min on the E. coli K-12 chromosome, was mapped genetically by Metzer et al. (22) and shown to contain four genes: gabT, encoding glutamate: succinic semialdehyde transaminase (GSST; GABA transaminase; EC 2.6.1.19); gabD, encoding succinic semialdehyde dehydrogenase (SSDH; EC 1.2.1.16); gabP, encoding GABA permease; and a control gene, gabC, coordinately regulating their expression. In a previous work, we purified and characterized a GABA transaminase from E. coli K-12 (26). The corresponding gene was cloned on a 1.6-kb DraI-BamnHI fragment and overexpressed in E. coli, and its identity with gabT could be demonstrated (3). The gabT gene was shown to be situated on a 3.8-kb SalI-BamHI fragment together with the endogenous promoter (3). Moreover, we presented a restriction map of a 15-kb Sall fragment containing most of the E. coli gab cluster (3). Metzer and Halpern recently also reported the cloning of the E. coli K-12 gab region and suggested that the gab genes are divergently transcribed by two different promoters (21).In this paper, we report the complete nucleotide sequence of the GABA transaminase gene, gabT, and discuss the evolutionary relatedness of GABA transaminase to other aminotransferases. Furthermore, we describe the cloning and characterization of gabD and provide evidence that gabT and gabD are transcribed from a common promoter upstream of the gabD gene. MATERIALS AND METHODSBacterial strains, media, enzymes, and chemicals. For the transformation and expression experiments, the E. coli DH1 (17) and JM103 (20)

Safeners enhance herbicide tolerance in crop plants but not in target weeds, thus improving herbicide selectivity. The safeners isoxadifen-ethyl and mefenpyr-diethyl protect cereal crops from sulfonyl urea herbicides in postemergence application. The two safeners were shown here to induce the cellular xenobiotic detoxification machinery in Arabidopsis thaliana when applied to leaves in a way mimicking field application. Gene expression profiling revealed the induction of 446 genes potentially involved in the detoxification process. Transgenic Arabidopsis plants expressing a reporter gene under control of a safenerresponsive maize promoter were used as a model system to study the safener signalling pathway. Reporter gene analysis in the tga2/3/5/6, sid2-2 and npr1 mutants as compared with the wild-type background showed that safener inducibility required TGA transcription factors and salicylic acid (SA) in a NON-EXPRESSOR of PR-1 (NPR1)-independent pathway converging on two as-1 promoter elements. For the majority of the safener-responsive Arabidopsis genes, a similar dependence on TGA transcription factors and/or SA was shown by gene expression profiling in wild-type plants as compared with the tga2/3/5/6 and sid2-2 mutants. Thirty-eight percent of the genes, however, were induced by safeners in a TGA/SA-independent manner. These genes are likely to be controlled by WRKY transcription factors and cognate W-boxes in their promoters.

We have determined the nucleotide sequences of two structural genes of the Escherichia coli gab cluster, which encodes the enzymes of the 4-aminobutyrate degradation pathway: gabD, coding for succinic semialdehyde dehydrogenase (SSDH, EC 1.2.1.16) and gabP, coding for the 4-aminobutyrate (GABA) transport carrier (GABA permease). We have previously reported the nucleotide sequence of the third structural gene of the cluster, gabT, coding for glutamate: succinic semialdehyde transaminase (EC 2.6.1.19). All three gab genes are transcribed unidirectionally and their orientation within the cluster is 5'-gabD-gabT-gabP-3'. gabT and gabP are separated by an intergenic region of 234-bp, which contains three repetitive extragenic palindromic (REP) sequences. The gabD gene consists of 1,449 nucleotides specifying a protein of 482 amino acids with a molecular mass of 51.7 kDa. The protein shows significant homologies to the NAD(+)-dependent aldehyde dehydrogenase (EC 1.2.1.3) from Aspergillus nidulans and several mammals, and to the tumor associated NADP(+)-dependent aldehyde dehydrogenase (EC 1.2.1.4) from rat. The permease gene gabP comprises 1,401 nucleotides coding a highly hydrophobic protein of 466 amino acids with a molecular mass of 51.1 kDa. The GABA permease shows features typical for an integral membrane protein and is highly homologous to the aromatic acid carrier from E. coli, the proline, arginine and histidine permeases from Saccharomyces cerevisiae and the proline transport protein from A. nidulans. Uptake of GABA was increased ca. 5-fold in transformants of E. coli containing gabP plasmids.(ABSTRACT TRUNCATED AT 250 WORDS)

Three hundred bacterial isolates from soil were tested for resistance against phosphinothricin [PPT; DL-homoalanin-4-yl(methyl)phosphinic acid], the active ingredient of the herbicide BASTA. Eight resistant bacterial strains and Escherichia coli were analyzed for PPT-transforming activities. At least three different enzymatic reactions could be detected in cell extracts. In six strains an acetyltransferase was active, synthesizing N-acetyl-PPT in the presence of PPT and acetyl coenzyme A. All strains could degrade PPT to its corresponding 2-oxoacid {2-oxo-4-[(hydroxy)(methyl)phosphinoyl] butyric acid} by transamination. Rhodococcus sp., the only tested strain that was able to utilize PPT as a sole source of nitrogen, formed 2oxo-4[(hydroxy)(methyl)phosphinoyl]butyric acid by oxidative deamination. This enzymatic activity was inducible by L-glutamic acid or PPT itself but not in the presence of NH4'. D-PPT transformation was not detectable in any of the investigated strains.

The staphylococcal enterotoxin serotype B (SEB)-induced enteric intoxication and the immediate-type reaction in the skin of unsensitized monkeys was used to define whether agents competing with SEB for target cell receptors may inhibit pathophysiological effects. For this purpose a duodenal provocation test was developed by use of a pediatric gastroscope, allowing the evaluation of the influence of antagonists on the intestinal disorder upon SEB challenge at the same duodenal site. First, carboxymethylation of histidine residues of SEB caused a complete loss of emetic and skinsensitizing activity without changing the immunological specificity. However, carboxymethylated SEB is a strong inhibitor of enteric intoxications and immediate-type skin reactions upon SEB challenge. Second, after immunization of BALB/c mice with monoclonal anti-SEB antibodies, monoclonal antiidiotypic antibodies (anti-Id) were obtained by the "hybridoma technique" and purification by idiotype-affinity chromatography. Anti-Id specifically inhibited the binding of horseradish peroxidase-labeled anti-SEB to the ligand, and SEB blocked as well the interaction of these two antibody species, indicating a high degree of binding-site selectivity. Anti-Id completely protected against emetic response and diarrhea upon duodenal provocation with SEB and inhibited immediate-type skin reactions as well. Further, anti-Id acted as an antagonist without triggering biologic functions themselves. This shows that anti-Id constitute a useful tool to protect against a bacterial toxininduced intestinal disorder.Staphylococcal enterotoxins (SE) are responsible for one of the most common types of food poisoning in humans (1). All SE produce emesis and diarrhea in humans and other primates as a result of oral administration, whereas the toxin appears to have little, if any, clinical effect in other laboratory animals (1). Although considerable efforts have been expended on attempts to define the pathogenesis, so far very little information has been available on the mode and cellular site of SE action in the gastrointestinal tract.Recently, however, evidence was provided that unsensitized monkeys develop an immediate-type reaction in the skin upon intradermal challenge with SE serotype B (SEB; ref.2). As shown by a series of experiments, SEB administered intradermally causes skin reactions by affecting mast cells (2). This type of nonimmunological mast cell stimulation by SEB offered a new approach, providing a model for investigating the mechanisms of SEB action. In addition, evidence was provided that carboxymethylation of SEB resulted in a loss of toxicity associated with the complete abrogation of skin-sensitizing activity without changing the immunological specificity of the toxin. It has been established that carboxymethylated SEB (CM-SEB) could compete with SEB for binding sites on the target-cell surface (2). To define whether SEB exerts its effect on mast cells by binding to specific celt-surface receptors or whether a less specific type of ligand-...