Discovery of pinoresinol reductase genes in sphingomonads

Bacterial genes for the degradation of major dilignols produced in lignifying xylem are expected to be useful tools for the structural modification of lignin in plants. For this purpose, we isolated pinZ involved in the conversion of pinoresinol from Sphingobium sp. strain SYK-6. pinZ showed 43–77% identity at amino acid level with bacterial NmrA-like proteins of unknown function, a subgroup of atypical short chain dehydrogenases/reductases, but revealed only 15–21% identity with plant pinoresinol/lariciresinol reductases. PinZ completely converted racemic pinoresinol to lariciresinol, showing a specific activity of 46 ± 3 U/mg in the presence of NADPH at 30 °C. In contrast, the activity for lariciresinol was negligible. This substrate preference is similar to a pinoresinol reductase, AtPrR1, of Arabidopsis thaliana; however, the specific activity of PinZ toward (±)-pinoresinol was significantly higher than that of AtPrR1. The role of pinZ and a pinZ ortholog of Novosphingobium aromaticivorans DSM 12444 were also characterized.