*4.4. Analytical Methods*

Cell densities of cultures were determined by measuring their absorbance at 600 nm with a 2800 UV/visible spectrophotometer [UNICO (Shanghai) INSTRUMENT]. For analysis of *trans*-cinnamic acid and *p*-coumaric acid, the reaction mixtures were centrifuged at 3500× *g* for 10 min, and supernatants were collected to analyze the product concentration by HPLC using a Wondasil C18 Superb column (250 mm × 4.6 mm, 5 µm) with the column temperature of 30 ◦C. To detect the formation of *trans*-cinnamic acid, we used a solution (50% of 1% acetic acid and 50% of acetonitrile) as mobile phase at flow rate of 0.7 mL/min. The detection wavelength was set at 290 nm. To analyze the production of *p*-coumaric acid, the HPLC was eluted with the solution (60% of 0.1% formic acid and 40% of methyl alcohol) as mobile phase at flow rate of 0.6 mL/min. The detection wavelength was set at 310 nm. Meanwhile, the supernatants after centrifugation were adjusted to pH 4.2 by HCl and allowed to stand at room temperature for 30 min, then the acidified supernatants were centrifuged again to separate the *trans*-cinnamic acid precipitate. The supernatants after centrifugation were adjusted to pH 4.6 by H3PO<sup>4</sup> to obtain *p*-coumaric acid and other steps were same as the isolation of *trans*-cinnamic acid.
