N-methyltransferase; part of the gene cluster that mediates the biosynthesis of benzomalvin A and D (PubMed:28604695). The pathway begins with the loading of amino acid precursors onto the A domains of the non ribosomal peptide synthetases benY and benZ (PubMed:28604695). BenY and the A1 domain of benZ are loaded with anthranilate (Anth), while the A2 domain of benZ is loaded with phenylalanine (Phe) (PubMed:28604695). N-methylation of Phe by the methyltransferase benX may happen before loading of Phe onto benZ, after loading of Phe, or after dipeptide formation (PubMed:28604695). Condensation of Anth with the secondary amine of NmPhe or Phe is catalyzed by the C1 domain of benZ, forming a dipeptide intermediate (PubMed:28604695). This is followed by in trans condensation of the Anth-NmPhe dipeptide with Anth bound to the T domain of benY by the C2 domain of benZ to form the linear tripeptide Anth-NmPhe-Anth (PubMed:28604695). Cyclization and release of the tripeptide is then catalyzed by the C-terminal C domain of benY and the resulting 11-member macrocyclic intermediate is expected to spontaneously collapse to form the benzodiazepine core (PubMed:28604695). Benzomalvin A is in conformational equilibrium with its atropisomer, benzomalvin D (PubMed:28604695). Secondary metabolite biosynthesis. Leads to a tenfold decrease in the amount of benzomalvins A and D, but not their complete abolishment. Belongs to the methyltransferase superfamily.