Functional investigation of the SAM-dependent methyltransferase RdmB in anthracycline biosynthesis

A novel sub-class of <i>S</i>-adenosyl-l-methionine (SAM)-dependent methyltransferases catalyze atypical chemical transformations in the biosynthesis of anthracyclines. Exemplified by RdmB from <i>Streptomyces purpurascens,</i> it was found with 10-decarboxylative hydroxylation activity on anthracyclines. We herein investigated the catalytic activities of RdmB and discovered a previously unknown 4-<i>O</i>-methylation activity. The site-directed mutagenesis studies proved that the residue at position R307 and N260 are vital for the decarboxylative hydroxylation and 4-<i>O</i>-methylation, respectively, which define two distinct catalytic centers in RdmB. Furthermore, the multifunctionality of RdmB activity was found as cofactor-dependent and stepwise. Our findings expand the versatility and importance of methyltransferases and should aid studies to enrich the structural diversity and bioactivities of anthracyclines.