A Self‐Sufficient β‐Methylarginine Biosynthetic Pathway in <i>Planctomycetes</i>

β-Methyl amino acids (β-MAAs) enhance the bioactivity of natural products and possess intrinsic pharmacological properties as free amino acids. While the biosynthetic capacity for this class of noncanonical amino acids has been established in certain bacterial lineages such as Gammaproteobacteria and Actinomycetes, other bacterial phyla remain largely unexplored. A genome-mining-guided discovery of a novel biosynthetic gene cluster capable of producing β-methylarginine, from the phylum Planctomycetes is reported. Both in vivo and in vitro evidence indicates that Planctomycetes employ a transaminase (PlaA) and a methyltransferase (PlaB) to synthesize this β-MAA. Unlike previously described β-methylarginine biosynthetic pathways, PlaA and PlaB function as a self-sufficient enzyme cascade that operates without the need for additional keto acid and amino acid partners. These findings expand the catalytic repertoire for β-MAAs biosynthesis and establish Planctomycetes as a new source of secondary metabolites discovery.