The main characteristics of all RiPPs are the PTMs. These are crucial, since an unmodified prepeptide lacks antimicrobial activity (Arnison et al., 2013). The natural variety of modifications is tremendous, from simple terminal decapping to complex ring formation in nisin (Wang, 2012). Complete chemical synthesis of nisin is feasible, however, since the molecular structure is highly complex, it suffers from a tedious multi-step synthesis and low yield (Fukase et al., 1988; Ongey and Neubauer, 2016). Therefore, in vivo methods for engineering RiPPs are required.