Taxonomy of Aureobasidium spp. and biosynthesis and regulation of their extracellular polymers

The genus Aureobasidium spp. have been divided into three species, A. pullulans, A. leucospermi and A. proteae, and A. pullulans has been known to have five varieties. However, after analysis of many strains of this yeast isolated from different environments, they do not belong to any of the three species or the five varieties. Although pullulan produced by A. pullulans has been widely used in different fields in industry and different strains of this yeast has been known to produce poly(beta-L-malic acid) (PMA), heavy oils and beta-1,3-glucan, it is still unknown how the black yeast synthesizes and secretes the extracellular polymers at molecular level. In this review article, new biosynthetic pathways of pullulan, PMA and heavy oils, the enzymes and their genes related to their biosynthesis and regulation are proposed. Furthermore, some enzymes and their genes related to pullulan biosynthesis in A. pullulans have been characterized. But it is completely unknown how pullulan is secreted and how PMA, heavy oils and beta-1,3-glucan are synthesized and secreted. Therefore, there is much work to be done about taxonomy and biosynthesis, secretion and regulation of pullulan, PMA, heavy oils and beta-1,3-glucan at molecular levels in Aureobasidium spp.