␥-Aminobutyrate (GABA) is metabolized to succinic semialdehyde by GABA aminotransferase (GABA-AT), and the succinic semialdehyde is subsequently oxidized to succinate by succinic semialdehyde dehydrogenase (SSADH). In Escherichia coli, there are duplicate GABA-ATs (GabT and PuuE) and duplicate SSADHs (GabD and YneI). While GabT and GabD have been well studied previously, the characterization and expression analysis of PuuE and YneI are yet to be investigated. By analyzing the amino acid profiles in cells of ⌬puuE and/or ⌬gabT mutants, this study demonstrated that PuuE plays an important role in GABA metabolism in E. coli cells. The similarity of the amino acid sequences of PuuE and GabT is 67.4%, and it was biochemically demonstrated that the catalytic center of GabT is conserved as an amino acid residue important for the enzymatic activity in PuuE as Lys-247. However, the regulation of expression of PuuE is significantly different from that of GabT. PuuE is induced by the addition of putrescine to the medium and is repressed by succinate and low aeration conditions; in contrast, GabT is almost constitutive. Similarly, YneI is induced by putrescine, while GabD is not. For E. coli, PuuE is important for utilization of putrescine as a sole nitrogen source and both PuuE and YneI are important for utilization of putrescine as a sole carbon source. The results demonstrate that the PuuE-YneI pathway was a putrescine-inducible GABA degradation pathway for utilizing putrescine as a nutrient source.

bIn Escherichia coli, putrescine is metabolized to succinate for use as a carbon and nitrogen source by the putrescine utilization pathway (Puu pathway). One gene in the puu gene cluster encodes a transcription factor, PuuR, which has a helix-turn-helix DNA-binding motif. DNA microarray analysis of an E. coli puuR mutant, in which three amino acid residues in the helix-turnhelix DNA binding motif of PuuR were mutated to alanine to eliminate DNA binding of PuuR, suggested that PuuR is a negative regulator of puu genes. Results of gel shift and DNase I footprint analyses suggested that PuuR binds to the promoter regions of puuA and puuD. The binding of wild-type PuuR to a DNA probe containing PuuR recognition sites was diminished with increasing putrescine concentrations in vitro. These results suggest that PuuR regulates the intracellular putrescine concentration by the transcriptional regulation of genes in the Puu pathway, including puuR itself. The puu gene cluster is found in E. coli and closely related enterobacteria, but this gene cluster is uncommon in other bacterial groups. E. coli and related enterobacteria may have gained the Puu pathway as an adaptation for survival in the mammalian intestine, an environment in which polyamines exist at relatively high concentrations.