Involved in the biosynthesis of petrobactin, a catecholate siderophore that functions in both iron acquisition and virulence (PubMed:17189355, PubMed:17346033). Aryl-carrier protein that activates 3,4-dihydroxybenzoate (3,4-DHBA) prior to its incorporation into petrobactin (PubMed:17346033). Siderophore biosynthesis; petrobactin biosynthesis. Activated by the transfer of a 4'-phosphopantetheine group from CoA to Ser-40. 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 acyl carrier protein (ACP) family.