| Summary: | The detection of vanillin during the metabolism of ferulic acid by several <i>Bacillus</i> strains has been reported; however, its occurrence is not yet understood. Herein, the potential enzymes involved in vanillin production during ferulic acid metabolism in the previously reported butanol-tolerant <i>Bacillus subtilis</i> strain GRSW1-B1 were explored. The recombinant <i>E. coli</i> cells that overexpressed phenolic acid decarboxylase (PadC) rapidly converted ferulic acid to 4-vinylguaiacol. The detection of vanillin was concurrent with a decrease in 4-vinylguaiacol. In addition, the reversible abiotic conversion of 4-vinylguaiacol and apocynol was observed. The overexpression of CypD, a <i>Bacillus</i> P450, resulted in notable production of vanillin. The two-step conversion of ferulic acid yielded 145 μM over 72 h at pH 9. Vanillin yields of approximately 258 μM and 212 μM were obtained from ferulic acid metabolism by recombinant <i>E. coli</i> coexpressing PadC and CypD after conversion for 72 h, at pH 9 and 10, respectively. Several possibilities that underlie the production of vanillin were discussed. This information is useful for understanding ferulic acid metabolism <i>by Bacillus</i> strains and for further improving this strain as a host for the production of valuable compounds from biomass.
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