KtzJ-dependent serine activation and O-methylation by KtzH for kutznerides biosynthesis

Kutznerides are hexadepsipeptide antifungal and antimicrobial agents containing O-methyl-L-serine in their very unique peptidic backbone. During kutznerides biosynthesis, this O-methylated amino-acid residue is proposed to result from the action of an adenylation (A) domain present in KtzH, which is interrupted by the S-adenosylmethionine-binding-containing part of a methyltransferase. In this study, we co-expressed recombinant KtzH(A4MA4T4) with its MbtH-like protein partner KtzJ and demonstrated the requirement for KtzJ in producing soluble and active KtzH(A4MA4T4). We demonstrated the specificity of KtzH(A4MA4T4) toward L-Ser and showed the activity of the partial methyltransferase enzyme in O-methylation of L-Ser after its covalent attachment to the thiolation domain of KtzH(A4MA4T4). The insights gained from this work may guide future study and development of engineered interrupted adenylation domains for combinatorial biosynthetic methodologies.