Involved in the initial steps of the mycobactin biosynthetic pathway. Putatively couples activated salicylic acid with serine or threonine and cyclizes this precursor to the hydroxyphenyloxazoline ring system present in this class of siderophores (By similarity). Binds 2 phosphopantetheines covalently. Siderophore biosynthesis; mycobactin biosynthesis. Modular protein that contains an aryl carrier protein (ArCP) domain which bears a phosphopantetheinyl arm to attach the activated salicylic acid, a condensation/cyclization domain involved in the formation of the oxazoline ring, an adenylation domain which activates the serine or threonine residue into an aminoacyl-AMP ester, a peptidyl carrier protein (PCP) domain which bears a phosphopantetheinyl arm to attach the activated serine or threonine, and a terminal thioesterase domain which assists in the transfer of intermediates from MbtB to ACP1 in MbtD. 4'-phosphopantetheine is transferred from CoA to a specific serine in each of the two carrier protein domains, leading to their activation from apo to holo forms. Belongs to the ATP-dependent AMP-binding enzyme family. MbtB subfamily.