AmXlnR, a transcription factor involved in xylan degradation and pentose catabolism, enhances pullulan production from xylose in Aureobasidium melanogenum

The abundant presence of xylan in low-cost lignocellulosic biomasses establishes its monomeric sugar, xylose, as a pivotal substrate for sustainable bioconversion processes. Although significant advances have been devoted to rewiring the metabolic pathways for xylose assimilation, the effort on regulatory dimensions remains scare. <i>Aureobasidium</i> spp., yeast-forming fungi known for their broad xylanolytic activities and high-yield production of value-added metabolites, represent promising microbial cell factories. However, the regulatory mechanism of xylan degradation and pentose catabolism in these yeasts has not been elucidated. In this study, we identified AmXlnR as the key transcriptional regulator governing xylan degradation and pentose catabolism (d-xylose and l-arabinose) in <i>Aureobasidium melanogenum</i>. Integrated transcriptomic, biochemical, and genetic analyses demonstrate that AmXlnR functions as an activator of xylanase and β-xylosidase genes, while functions as a repressor of α-l-arabinofuranosidase genes, through recognition of specific promoter motifs. Furthermore, AmXlnR positively influences pullulan biosynthesis from pentoses by significantly upregulating the pentose catabolic pathway. Notably, overexpression of <i>AmXLNR</i> alone enhanced pullulan production from xylose, achieving a titer of 56.75 ± 1.51 g/L, which is comparable to the yield obtained with glucose as the carbon source. These findings establish AmXlnR as a key regulator of xylanolytic gene expression in <i>A. melanogenum</i> and provide a new strategic approach to improve bioconversion of lignocellulosic biomass-derived pentose for high-value product synthesis.