Modular nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) enzymatic assembly lines are large and dynamic protein machines that generally undergo a linear progression of catalytic cycles via a series of enzymatic domains organized into independent modules. Here we report the heterologous reconstitution and comprehensive characterization of two hybrid NRPS-PKS assembly lines that defy many standard rules of assembly-line biosynthesis to generate a large combinatorial library of cyclic lipodepsipeptide protease inhibitors called thalassospiramides. We generate a series of precise domain-inactivating mutations in thalassospiramide assembly lines and present compelling evidence for an unprecedented biosynthetic model that invokes inter-module substrate activation and tailoring, module skipping, and pass-back chain extension, whereby the ability to pass the growing chain back to a preceding module is flexible and substrate-driven. Expanding bidirectional inter-module domain interactions could represent a viable mechanism for generating chemical diversity without increasing the size of biosynthetic assembly lines and raises new questions regarding our understanding of the structural features of multi-modular megaenzymes. Nonribosomal peptide synthetase (NRPS) and polyketide synthase (PKS) enzymes are molecular-scale assembly lines that construct complex polymeric products, many of which are useful to humans as life-saving drugs. The first characterized assembly lines exhibited an elegant co-linear biosynthetic logic, whereby the linear arrangement of functional units, called modules, along an NRPS/PKS polypeptide directly correlates to the chemical structure of the product 1 . The polyketide antibiotic erythromycin 2 and the nonribosomal peptide antibiotic daptomycin 3 are two such examples. The core components of an assembly line elongation module include adenylation (A) or acyltransferase (AT) domains for substrate selection, thiolation (T) or carrier protein domains for covalent substrate tethering, and condensation (C) or ketosynthase (KS) domains for chain extension. Optional tailoring domains such as methyltransferase (MT), ketoreductase (KR), or dehydratase (DH)domains, if present, chemically modify building blocks or chain-extension intermediates. This one-to-one correlation between product residues and assembly line modules with the requisite catalytic domains is one feature that makes NRPS/PKS enzymes among the largest proteins found within the tree of life.