a, Comparative genomic analysis of S. achromogenes var. achromogenes NRRL B-2120, a non-producer of SZN and S. achromogenes var. streptozoticus NRRL 2697 using Mauve28. The szn gene cluster is coloured in red. Gene annotations are tabulated in Supplementary Table 1. b, Multiple sequence alignment of SznE with structurally characterized protein arginine methyl transferases (PRMT) from eukaryotes. Conserved residues involved in binding l-arginine are marked with an asterisk. c, Overlay of a homology model of SznE (green) with the crystal structure of PRMT7 from T. brucei (PDB: 4M37) (orange). The highlighted carboxylate residues are involved in binding of the basic guanidine group15. d, SDS–PAGE of purified SznE. The expected molecular weight is 40 kDa. Ladder = Precision Plus Protein All Blue Standards (BioRad). e, Mass spectra of SZN produced when feeding S. achromogenes var. streptozoticus NRRL with d3-l-NMA. The expected masses [M − H2O + H]+ for SZN and d3-SZN are 248.0883 and 251.1060, respectively. f, LC–MS traces demonstrating the restoration of SZN production by the ΔsznE mutant upon chemical complementation with l-NMA. The EICs are generated within a 5 p.p.m. window.