Nonribosomal peptide synthetase; part of the gene cluster that mediates the biosynthesis of imizoquins A to D, tripeptide-derived alkaloids that serve a protective role against oxidative stress that are essential for normal germination (PubMed:29182847). ImqB is a canonical three-module NRPS that assembles the tripeptide backbone of the imizoquins via condensation of Trp, Tyr, and Leu-derived precursors (PubMed:29182847). N-methylation by imqF and phenol oxidation by imqC, followed by cyclization via the FAD-dependent oxidase imqH carry out the three-step transformation of L-tyrosine into tetrahydroisoquinoline (PubMed:29182847). Importantly, this sequence requires the presence of a free amine in the tyrosine moiety, indicating that isoquinoline formation occurs prior to peptide bond formation (PubMed:29182847). The imidazolidin-4-one ring of imizoquins could form following additional oxidation of the methyl-derived bridgehead carbon by imqH (PubMed:29182847). Lastly, O-methylation by imqG and leucine hydroxylation by imqE complete biosynthesis of the imizoquins (PubMed:29182847). Secondary metabolite biosynthesis. Expression is down-regulated by ralstonins, lipopeptides produced by the plant pathogenic bacteria Ralstonia solanacearum (PubMed:29182847). Expression is positively regulated by the imizoquins cluster-specific transcription regulator imqK (PubMed:29182847). NRP synthetases are composed of discrete domains (adenylation (A), thiolation (T) or peptidyl carrier protein (PCP) and condensation (C) domains) which when grouped together are referred to as a single module (By similarity). Each module is responsible for the recognition (via the A domain) and incorporation of a single amino acid into the growing peptide product (By similarity). Thus, an NRP synthetase is generally composed of one or more modules and can terminate in a thioesterase domain (TE) that releases the newly synthesized peptide from the enzyme (By similarity). Occasionally, epimerase (E) domains (responsible for L- to D-amino acid conversion) are present within the NRP synthetase (By similarity). ImqB has the following architecture: A-T-C-A-T-C-A-T-TE (PubMed:29182847). Belongs to the NRP synthetase family.