S-Adenosylmethionine (SAM) is a well-known methyl donor for the methylation of nucleic acids,proteins,and small molecules in both prokaryotes and eukaryotes.Our previous studies have revealed its novel function as a widespread activator for antibiotic biosynthesis in various Streptomyces species although the underlying molecular events remain unclear.In this study,we studied the mechanism by which SAM activated actinorhodin production in S.coelicolor A3(2) using afsK-disruptant and its complemented strains,including complementation by full length afsK gene or its 1-933 bp N-terminal fragment (afsKN),under the control of the native promoter.Activation of actinorhodin production by SAM treatment was observed both in afsK and afsKN complementation strains.Promoter activity of afsK was not apparently affected by exogenously applied SAM when examined using a promoter-probe vector.Further examination on the in vitro autophosphorylation of AfsK and AfsKN substantiated that SAM activated the in vitro autophosphorylation of both AfsK and AfsKN.Our results have suggested the important role of N-terminus kinase domain of AfsK in SAM-mediated activation of actinorhodin production in S.coelicolor A3(2).