Bioconversion of ferulic acid into vanillic acid by means of a vanillate-negative mutant of Pseudomonas fluorescens strain BF13.

From a ferulic-acid-degrading Pseudomonas fluorescens strain (BF13), we have isolated a transposon mutant, which retained the ability to bioconvert ferulic acid into vanillic acid but lost the ability to further degrade the latter acid. The mutant, BF13-97, was very stable, and therefore it was suitable to be used as a biocatalyst for the preparative synthesis of vanillic acid from ferulic acid. By use of resting cells we determined the effect on the bioconversion rate of several parameters, such as the addition of nutritional factors, the concentration of the biomass, and the carbon source on which the biomass was grown. The optimal yield of vanillic acid was obtained with cells pregrown on M9 medium containing p-coumaric acid (0.1% [wt/vol]) as a sole carbon source and yeast extract (0.001% [wt/vol]) as a source of nutritional factors. Under these conditions, 1 mg (wet weight) of biomass produced 0.23 mg of vanillic acid per h. The genomic region of BF13-97 flanking the transposon's site of insertion was cloned and sequenced revealing two open reading frames of 1,062 (vanA) and 954 (vanB) bp, respectively. The van genes are organized in a cluster and encode the subunits of the vanillate-O-demethylase, which catalyzes the first step of the vanillate catabolism. Amino acid sequences deduced from vanA and vanB genes were shown to have high identity with known VanAs and VanBs from Pseudomonas and Acinetobacter spp. Highly conserved regions known to exist in class IA oxygenases were also found in the vanillate-O-demethylase components from P. fluorescens BF13. The terminal oxygenase VanA is characterized by a conserved Rieske-type [2Fe-2S](R) ligand center. The reductase VanB contains a plant-type ferredoxin [2Fe-2S](Fd), flavin mononucleotide, and NAD-ribose binding domains which are located in its C-terminal and N-terminal halves, respectively. Transfer of wild-type vanAB genes to BF13-97 complemented this mutant, which recovered its ability to grow on either vanillic or ferulic acid.