Characterization of and functional evidence for Ste27 of Streptomyces sp. 139 as a novel spermine/spermidine acetyltransferase
Ebosin, a novel exopolysaccharide produced by Streptomyces sp. 139, has remarkable anti-rheumatoid arthritis activity in vivo and its biosynthesis gene cluster (ste) consists of 27 ORFs (open reading frames). The present paper reports our study of the protein product encoded by ste27. Database searching reveals the homology of Ste27 with some spermidine/spermine acetyltransferases. To confirm the prediction, the ste27 gene was cloned and expressed in Escherichia coli BL21(DE3) cells and recombinant Ste27 was purified. The following enzymatic analysis revealed its ability of transferring the acetyl group from acetyl-CoA to spermidine and spermine, with spermidine being the preferred substrate. Ste27 can acetylate the N1, N4 and N8 positions on spermidine. The Km values of Ste27 were determined for spermidine and spermine, as well as for acetyl-CoA, poly-L-lysine and glucosamine 6-phosphate. Upon gene knockout, the exopolysaccharide-27m produced by the mutant strain Streptomyces sp. 139 (ste27-), compared with Ebosin, exhibited a significantly reduced binding activity to the interleukin-1 receptor. After gene complementation, the binding activity was partially restored. This demonstrated that the ste27 gene is involved in the biosynthesis of Ebosin. Molecular modelling was also carried out to predict the binding mode of Ste27 with acetyl-CoA, spermidine or spermine.