Identification of the protocatechuate transporter gene in Sphingobium sp. strain SYK-6 and effects of overexpression on production of a value-added metabolite.

Sphingobium sp. strain SYK-6 expresses the best characterized catabolic systems for lignin-derived aromatic compounds. However, the uptake systems for these aromatics remain unknown. In this study, we identified and characterized the protocatechuate (PCA) transporter gene SLG_12880 (pcaK) in SYK-6. Sequence comparisons revealed that PcaK belongs to the aromatic acid/H symporter family of major facilitator superfamily transporters. Further, pcaK was highly conserved among Sphingomonadales possessing catabolic genes for vanillate and PCA. The growth of an SYK-6 pcaK mutant was significantly delayed on PCA medium and PCA uptake rate was only 8% of wild type. In addition, vanillate uptake rate was 78% of wild type, although the pcaK mutant grew as well as the wild type on vanillate. When pcaK was expressed in Sphingobium japonicum UT26S, the transformant acquired the capacity to uptake both PCA and vanillate. These results indicate that pcaK is responsible for the major proportion of PCA uptake and a minor fraction of vanillate uptake in SYK-6. The productivity of 2-pyrone-4,6-dicarboxylate (PDC), a building block of functional polymers, was evaluated using a PDC hydrolase SYK-6 mutant harboring a pcaK plasmid. The mutant exhibited 1.27-fold greater PCA conversion and 1.24-fold greater PDC production compared to the control strain, suggesting that enhanced expression of transporter genes for lignin-derived aromatics can be used to increase the production of value-added metabolites.