Decarboxylation of sorbic acid to 1,3-pentadiene has been demonstrated in several mould species, including Trichoderma and Penicillium spp.

and in a few yeast species ( Marth et al., 1966, Kurogochi et al., 1975, Kinderlerler and Hutton, 1990, Casas et al., 1999, Casas et al., 2004 and Pinches and Apps, 2007). The activity of a cinnamic acid decarboxylase, encoded by the gene padA1 (PAD1 in the yeast Saccharomyces cerevisiae) ( Clausen et al., 1994) is responsible for the decarboxylation of both sorbic and cinnamic acids in Epigenetic Reader Domain inhibitor germinating spores of A. niger ( Plumridge et al., 2008). Alternative names for cinnamic acid include phenylacrylic acid ( Clausen et al., 1994) but more correctly, 3-phenyl-(E)-2-propenoic acid or tert-β-phenylacrylic acid ( Burdock, 2002). Disruption of the padA1 gene resulted in 50% lower concentrations of sorbic acid to prevent conidial outgrowth. In contrast, in the yeast S. cerevisiae, PAD1 activity is slight and gene disruption did not alter resistance to sorbic acid ( Stratford et al., 2007) demonstrating that Pad activity did not contribute to preservative resistance in that yeast. The view that decarboxylase activity depended solely on the induction of pad genes MK 1775 was shown

to be an over-simplification by the discovery ( Plumridge et al., 2010) that the decarboxylation process in A. niger also requires activity of a putative 2-hydroxybenzoic acid decarboxylase, encoded by ohbA1 (3-octaprenyl-2-hydroxybenzoic acid decarboxylase) and a putative transcription factor encoded by sdrA (sorbic acid decarboxylase regulator). These three genes, padA1, ohbA1 and sdrA, form a cluster on chromosome 6 in A. niger. Two other homologous clusters, padA2/ohbA2 and padA3/ohbA3, are present

at other loci in the A. niger genome but are not expressed in the presence of sorbic acid. Further bioinformatic analysis showed that this clustering was highly conserved in several Aspergillus species and also, with the exception of a homologue of sdrA, in the yeast S. cerevisiae ( Mukai et al., Calpain 2010). This conserved synteny indicates a clustering of metabolic function and regulation, although the role of the PadA1 and OhbA1 proteins, together or in sequence in the decarboxylation process (referred to subsequently as the Pad-decarboxylation system), remains to be revealed. The objectives of this study were to identify the structural features of chemicals that transcriptionally induce the Pad-decarboxylation system in developing conidia of A. niger and to define the structural features that determine the substrate acceptability by the decarboxylase system. The (unknown) complexity of the Pad-decarboxylation system mitigates against the use of X-ray crystallography although there are crystal structures of purified Pad-decarboxylases from Escherichia coli (Protein Data Bank, PDB, entry 1sbz; Rangarajan et al., 2004) and Aquifex aeolicus (PDB entry 2ejb).