Pullulan biosynthesis in yeast-like fungal cells is regulated by the transcriptional activator Msn2 and cAMP-PKA signaling pathway

Pullulan is an important polysaccharide. Although its synthetic pathway in Aureobasidium melanogenum has been elucidated, the mechanism underlying its biosynthesis as regulated by signaling pathway and transcriptional regulator is still unknown. In this study, it was found that the expression of the UGP1 gene encoding UDPG-pyrophosphorylase (Ugp1) and other genes which were involved in pullulan biosynthesis was controlled by the transcriptional activator Msn2 in the nuclei of yeast-like fungal cells. The Ugp1 was a rate-limiting enzyme for pullulan biosynthesis. In addition, the activity and subcellular localization of the Msn2 were regulated only by the cAMP-PKA signaling pathway. When the cAMP-PKA activity was low, the Msn2 was localized in the nuclei, the UGP1 gene was highly expressed, and pullulan was actively synthesized. By contrast, when the cAMP-PKA activity was high, the Msn2 was localized in the cytoplasm and the UGP1 gene expression was disabled so that pullulan was stopped, but lipid biosynthesis was actively enhanced. This study was the first to report that pullulan and lipid biosynthesis in yeast-like fungal cells were regulated by the Msn2 and cAMP-PKA signaling pathway. Elucidating the regulation mechanisms was important to understand their functions and enhance pullulan and lipid biosynthesis.