A UDP-glycosyltransferase from Bacillus licheniformis was exploited for the glycosylation of phloretin. The in vitro glycosylation reaction confirmed the production of five phloretin glucosides, including three novel glucosides. Consequently, we demonstrated the application of the same glycosyltransferase for the efficient whole-cell biocatalysis of phloretin in engineered Escherichia coli. P hloretin is a dihydrochalcone, an intermediate of the biosynthetic pathway of flavonoids in plants, which is abundantly present in the peel of apple (1, 2) and in strawberries (3). They occur in different glycosidic forms, such as naringin dihydrochalcone, phlorizin, and phloretin-4=-O-glucoside, in the different parts of the plants, contributing to various physiological properties of the plants, as well as to their color. Phloretin and its glycosides have been determined to have beneficial biological activities. Studies have uncovered that phloretin has inhibitory activity against glucose cotransporter 1 (4, 5), antioxidant activity (6), and activity to suppress the tumor necrosis factor alpha-induced inflammatory response, ameliorate inflammation of the colon, positively affect body weight loss (7), modulate Ca 2ϩ -activated K ϩ channels, and increase endothelial nitric oxide production, which might help to protect against atherosclerosis (8). Importantly, phloretin has other biological functions, like anticarcinogenic (9) and estrogenic activities (10) and inhibition of cardiovascular disease (11, 12). Irrespective of their diverse physiological and pharmacological activities, the use of most of the natural polyphenols as drugs and food additives has been limited because of their water insolubility and low absorbability. Glycosylation enhances the bioavailability and pharmacological properties of compounds by increasing their solubility and stability (13,14). Importantly, the sugar moieties of the glycosides often participate in the specific recognition of their biological targets and help to determine their efficacy in drug development (14, 15). According to the CAZy database (http://www .cazy.org/) (16,17,18), glycosyltransferase family 1 (GT1) proteins contain the UDP-glycosyltransferases that are common in all domains of life (19) and predominantly recognize small molecules as the sugar acceptors. A recent report showed that YjiC, a Bacillus licheniformis UDP-glycosyltransferase that falls in the GT1 family of proteins, can glycosylate at different hydroxyl positions of geldanamycin analogs (20). Here, we report the use of this glycosyltransferase for the biosynthesis of diverse phloretin glucosides in vitro and the subsequent application of YjiC for in vivo production of phloretin glucosides in an Escherichia coli mutant generating a cytoplasmic pool of UDP-glucose, since the YjiC-homologous glycosyltransferases from other Bacillus species were found to have flexible glycosyltransferase activities toward different flavonoid groups of compounds. Moreover, we found that by reversing the glycosylation reaction, the enz...