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Gab operon

The gab operon is responsible for the conversion of γ-aminobutyrate (GABA) to succinate. The gab operon comprises three structural genes – gabD, gabT and gabP – that encode for a succinate semialdehyde dehydrogenase, GABA transaminase and a GABA permease respectively. There is a regulatory gene csiR, downstream of the operon, that codes for a putative transcriptional repressor and is activated when nitrogen is limiting.

The gab operon has been characterized in Escherichia coli and significant homologies for the enzymes have been found in organisms such as Saccharomyces cerevisiae, rats and humans.

Limited nitrogen conditions activate the gab genes. The enzymes produced by these genes convert GABA to succinate, which then enters the TCA cycle, to be used as a source of energy. The gab operon is also known to contribute to polyamine homeostasis during nitrogen-limited growth and to maintain high internal glutamate concentrations under stress conditions.

Structure

The gab operon consists of three structural genes:

gabT : encodes a GABA transaminase that produces succinic semialdehyde.
gabD : encodes an NADP-dependent succinic semialdehyde dehydrogenase, which oxidizes succinic semialdehyde to succinate.
gabP : encodes a GABA-specific permease.

Physiological significance of the operon

The gabT gene encodes for GABA transaminase, an enzyme that catalyzes the conversion of GABA and 2-oxoglutarate into succinate semialdehyde and glutamate. Succinate semialdehyde is then oxidized into succinate by succinate semialdehyde dehydrogenase which is encoded by the gabP gene, thereby entering the TCA cycle as a usable source of energy. The gab operon contributes to homeostasis of polyamines such as putrescine, during nitrogen-limited growth. It is also known to maintain high internal glutamate concentrations under stress conditions.

Regulation

Differential regulation of promoters

The expression of genes in the operon is controlled by three differentially regulated promoters, two of which are controlled by RpoS encoded sigma factor σS.

csiDp : is σS-dependent and is activated exclusively upon carbon starvation because cAMP-CRP acts an essential activator for σS containing RNA polymerase at the csiD promoter.
gabDp1: is σS -dependent and is induced by multiple stresses.
gabDp2: is σ70 dependent and is controlled by Nac (Nitrogen Assimilation Control) regulatory proteins expressed under nitrogen limitation.

Mechanism of regulation

References

(2002). "The ''Escherichia coli gabDTPC'' Operon: Specific γ-Aminobutyrate Catabolism and Nonspecific Induction". Journal of Bacteriology.
(1990). "Molecular Analysis of Two Genes of the ''Escherichia coli gab'' Cluster: Nucleotide Sequence of the Glutamate:Succinic Semialdehyde Transaminase Gene (''gabT'') and Characterization of the Succinic Semialdehyde Dehydrogenase Gene (''gabD'')". Journal of Bacteriology.
(2003). "Multiple stress signal integration in the regulation of the complex σS-dependent ''csiD-ygaF-gabDTP'' operon in ''Escherichia coli''". Molecular Microbiology.
(2004). "Activation of the ''gab'' Operon in an RpoS-Dependent Manner by Mutations That Truncate the Inner Core of Lipopolysaccharide in ''Escherichia coli''". Journal of Bacteriology.

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gene-expression operons

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