UDP-glucose epimerase 1,moonlighting as a transcriptional activator,is essential for tapetum degradation and male fertility in rice

Multiple enzymes perform moonlighting functions distinct from their main roles.UDP-glucose epimerases(UGEs),a subclass of isomerases,catalyze the interconversion of UDP-glucose(UDP-Glc)and UDP-galactose(UDP-Gal).We identified a rice male-sterile mutant,osuge1,with delayed tapetum degradation and abortive pollen.The mutant osuge1 protein lacked UDP-glucose epimerase activity,resulting in higher UDP-Gal content and lower UDP-Glc levels in the osuge1 mutant compared with the wild type.Interestingly,we discovered that OsUGE1 participates in the TIP2/bHLH142–TDR–EAT1/DTD transcriptional regulatory cascade involved in tapetum degradation,in which TIP2 and TDR regulate the expression of OsUGE1 while OsUGE1 regulates the expression of EAT1.In addition,we found that OsUGE1 regulates the expression of its own gene by directly binding to an E-box element in the OsUGE1 promoter.Collectively,our results indicate that OsUGE1 not only functions as a UDP-glucose epimerase but also moonlights as a transcriptional activator to promote tapetum degradation,revealing a novel regulatory mechanism of rice reproductive development.

Insights into the biosynthesis of septacidin L-heptosamine moiety unveils a VOC family sugar epimerase

L-Heptopyranoses are important components of bacterial polysaccharides and biological active secondary metabolites like septacidin(SEP),which represents a group of nucleoside antibiotics with antitumor,antifungal,and pain-relief activities.However,little is known about the formation mechanisms of those L-heptose moieties.In this study,we deciphered the biosynthetic pathway of the L,L-gluco-heptosamine moiety in SEPs by functional characterizing four genes and proposed that SepI initiates the process by oxidizing the 4’-hydroxyl of L-glycero-α-D-manno-heptose moiety of SEP-328(2)to a keto group.Subsequently,SepJ(C5 epimerase)and SepA(C3 epimerase)shape the 4’-keto-L-heptopyranose moiety by sequential epimerization reactions.At the last step,an aminotransferase SepG installs the 4’-amino group of the L,L-gluco-heptosamine moiety to generate SEP-327(3).An interesting phenomenon is that the SEP intermediates with 4’-keto-L-heptopyranose moieties exist as special bicyclic sugars with hemiacetal-hemiketal structures.Notably,L-pyranose is usually converted from D-pyranose by bifunctional C3/C5 epimerase.SepA is an unprecedented monofunctional L-pyranose C3 epimerase.Further in silico and experimental studies revealed that it represents an overlooked metal dependent-sugar epimerase family bearing vicinal oxygen chelate(VOC)architecture.