The R2R3-MYB transcription factor MYB44 modulates carotenoid biosynthesis in Ulva prolifera

Carotenoids compose a large group of natural terpenoid pigments and are widely distributed in algae. Carotenoids play key roles in many biological processes in Ulva prolifera, and the carotenoid biosynthetic pathway has been thoroughly elucidated; however, the mechanisms through which carotenoid biosynthesis is regulated by transcription factors are not fully understood, especially in algae. In this study, we reported that the R2R3-MYB transcription factor UpMYB44 is a positive regulator of carotenoid biosynthesis in U. prolifera. The full-length coding DNA sequence of UpMYB44 is 1437 bp and encodes a putative 478-amino acid protein. The UpMYB44 protein is located in the nucleus in U. prolifera and has transcriptional activity. UpMYB44 can regulate carotenoid accumulation by directly interacting with the key carotenoid synthesis gene UpPDS and can enhance activity under some environmental stresses, including temperature, light and salinity stresses, resulting in an increase in total carotenoids, β-carotene and lutein. Using yeast one-hybrid assays and electrophoretic mobility shift assays, we demonstrated that UpMYB44 directly binds to the cis-elements of the promoter of UpPDS. The results indicated that the expression level of the key gene involved in carotenoid biosynthesis affected the biosynthesis of carotenoids in U. prolifera at the molecular level and was regulated by UpMYB44. Overall, we characterized the U. prolifera transcription factor UpMYB44, which was found to activate expression of the key gene that plays an important role in the carotenoid biosynthetic pathway in U. prolifera. The data presented in this study suggest a role of UpMYB44 in carotenoid metabolism exerted by transcriptional activation of carotenoid biosynthetic pathway genes. These results further support recent progress in elucidating the roles of v-myb avian myeloblastosis viral oncogene homolog transcription factors in carotenoid regulation in algae.