Involved in the biosynthesis of petrobactin, a catecholate siderophore that functions in both iron acquisition and virulence (PubMed:16875672, PubMed:17189355, PubMed:22408253). Catalyzes the ATP-dependent condensation of spermidine with N(8)-citryl-spermidine or N(1)-(3,4-dihydroxbenzoyl)-N(8)-citryl-spermidine, two intermediates in petrobactin biosynthesis pathway (PubMed:18758617, PubMed:22408253). ATP + N(8)-citryl-spermidine + spermidine = AMP + diphosphate + H(+) + N(8),N'(8)-citryl-bis(spermidine) ATP + N(1)-(3,4-dihydroxybenzoyl)-N(8)-citryl-spermidine + spermidine = AMP + diphosphate + H(+) + N(1)-(3,4-dihydroxybenzoyl)-N(8),N'(8)-citryl-bis(spermidine) Siderophore biosynthesis; petrobactin biosynthesis. Homodimer. The deletion mutant cannot produce petrobactin on iron-depleted medium. In vitro analysis show that mutants grow to a very limited extent as vegetative cells in iron-depleted medium but are not able to outgrow from spores under the same culture conditions. Belongs to the IucA/IucC family.