Weak Acid Sensitive A, a novel transcription factor required for regulation of weak-acid resistance and spore-spore heterogeneity in Aspergillus niger

Propionic, sorbic and benzoic acids are organic weak acids that are widely used as food preservatives, where they play a critical role in preventing microbial growth. In this study, we uncovered new mechanisms of weak acid resistance in moulds. By screening a library of 401 transcription-factor deletion strains in Aspergillus fumigatus for sorbic acid hypersensitivity, a previously uncharacterised transcription factor was identified, and named as WasA (Weak Acid Sensitive A). The orthologous gene in the spoilage mould Aspergillus niger was identified and deleted. WasA was required for resistance to a range of weak acids, including sorbic, propionic and benzoic acids. A transcriptomic analysis was performed to characterise genes regulated by WasA during sorbic acid treatment in A. niger. Several genes were significantly upregulated in the wild type compared with a {Delta}wasA mutant, including genes encoding putative weak acid detoxification enzymes and transporter proteins. Among these was An14g03570, a putative ABC-type transporter which we found to be required for weak acid resistance in A. niger. We also show that An14g03570 is a functional homologue of the Saccharomyces cerevisiae protein Pdr12p. Lastly, resistance to sorbic acid was found to be highly heterogeneous within genetically-uniform populations of ungerminated A. niger conidia, and we demonstrate that An14g03570 is a determinant of this heteroresistance. This study has identified novel mechanisms of weak acid resistance in A. niger which could help to inform and improve future food spoilage prevention strategies.