NRPS-independent siderophore synthetase that catalyzes the rhizoferrin biosynthesis from citrate and diaminobutane via an ATP-dependent condensation of citrate with diaminobutane followed by the addition of a second citrate to the monocitryl-diaminobutane intermediate (PubMed:28610916). Can also use as substrates the citrate and diaminobutane homologs oxaloacetic acid, diaminopropane, diaminobutane, diaminopentane, tricarballylic acid, hydroxylamine and ornithine (PubMed:28610916). Forms only a mono-substituted intermediate with oxaloacetic acid and diaminopentane whereas both mono-citryl intermediates and full rhizoferrin derivatives were detected when diaminopropane, and ornithine were used as substrates (PubMed:28610916). Tricarballylic acid only forms a rhizoferrin derivative, but no mono-substituted intermediate (PubMed:28610916). Expression is repressed in iron-replete media (PubMed:28610916).