A. TruD and PatD bind to RSI and catalyze heterocycle formation. TruD, ~100% identical to PatD in the RRE and E1-like domain, is 77% identical to PatD in YcaO domain. PatD synthesizes thiazoline and oxazoline, while TruD is a thiazoline specialist. As defined by Walsh, thiazoline is energetically more favorable (Belshaw et al., 1998). B. TruD converts ATP to AMP and PPi, whereas some other heterocyclases such as those involved in linear azol(in)e-containing peptides (LAP) or thiopeptide biosynthesis yield ADP and Pi as the products.