Eukaryotic-type aromatic amino acid decarboxylase from the root colonizer Pseudomonas putida is highly specific for 3,4-dihydroxyphenyl-L-alanine, an allelochemical in the rhizosphere.

Aromatic amino acid decarboxylases (AADCs) are found in various organisms and play distinct physiological roles. AADCs from higher eukaryotes have been well studied because they are involved in the synthesis of biologically important molecules such as neurotransmitters and alkaloids. In contrast, bacterial AADCs have received less attention because of their simplicity in physiology and in target substrate (tyrosine). In the present study, we found that Pseudomonas putida KT2440 possesses an AADC homologue (PP_2552) that is more closely related to eukaryotic enzymes than to bacterial enzymes, and determined the genetic and enzymic characteristics of the homologue. The purified enzyme converted 3,4-dihydroxyphenyl-l-alanine (DOPA) to dopamine with K(m) and k(cat) values of 0.092 mM and 1.8 s(-1), respectively. The enzyme was essentially inactive towards other aromatic amino acids such as 5-hydroxy-l-tryptophan, l-phenylalanine, l-tryptophan and l-tyrosine. The observed strict substrate specificity is distinct from that of any AADC characterized so far. The proposed name of this enzyme is DOPA decarboxylase (DDC). Expression of the gene was induced by DOPA, as revealed by quantitative RT-PCR analysis. DDC is encoded in a cluster together with a LysR-type transcriptional regulator and a major facilitator superfamily transporter. This genetic organization is conserved among all sequenced P. putida strains that inhabit the rhizosphere environment, where DOPA acts as a strong allelochemical. These findings suggest the possible involvement of this enzyme in detoxification of the allelochemical in the rhizosphere, and the potential occurrence of a horizontal gene transfer event between the pseudomonad and its host organism.