Genome analysis of the obligate marine actinomycetes Salinispora tropica (Udwary et al., 2007) and Salinispora arenicola (Penn et al., 2009) suggested that they possess multiple siderophore-like PD-332991 biosynthetic loci. Four pathways are predicted in S. tropica CNB-440, whereas only two are retained in S. arenicola CNS-205. Both species maintain a des locus that likely codes for desferrioxamine
(DFO) and a sid2 locus related to the gene cluster for yersiniabactin biosynthesis, ybt (Gehring et al., 1998). Intriguingly, ybt is usually encoded on a high pathogenicity island that mobilizes between pathogenic Gram-negative bacteria to confer virulence (Buchrieser et al., 1998; Schubert et al., 1998; Flannery et al., 2009). Salinispora tropica CNB-440 also encodes two additional nonribosomal peptide synthetase (NRPS) pathways, sid3 and sid4, which
are hypothesized to provide unique salicylate-containing iron chelators similar to dihydroaeruginoic acid (Carmi & Carmeli, 1994) and the predicted ‘coelibactin’ (Bentley et al., 2002). DFOs are hydroxamate-type siderophores with a high affinity for iron (Kd ~ 10−31 M) (Keberle, 1964) that are produced by streptomycetes (Müller & Raymond, 1984; Barona-Gómez et al., 2004) and some Gram-negative bacteria (Martinez et al., 2001; Essén et al., 2007). Several analogs have been reported including Selleck Screening Library linear DFOs B, D and G and cyclic DFO E (Fig. 3a), as well as acyl-DFO analogs with terminal branched alkyl chains or aromatic rings (D’Onofrio et al., 2010; Yang et al., 2011). DFOs are biosynthesized via an NRPS-independent mechanism (Challis, Mephenoxalone 2005), encoded by desA-D (Barona-Gómez et al., 2004; Kadi et al., 2007). Transcription from des is repressed by the divalent metal-dependent regulatory protein DmdR1 and derepressed by iron limitation (Flores & Martín, 2004; Tunca et al., 2007). Predicted homologs to desA-D and the ferric-siderophore uptake and utilization genes (desE-F)
are found in both Salinispora genomes (Fig. 1a). Despite bioinformatic predictions on the siderophores produced by Salinispora, no iron chelators have been isolated from this genus. Therefore, we explored the siderophore chemistry of these marine actinomycetes to determine which of the putative siderophore biosynthetic loci play a role in iron acquisition in Salinispora. Salinispora tropica strain CNB-440, S. arenicola strains CNS-205, CNT-088 and CNH-643 and ‘Salinispora pacifica’ strain CNT-133 were cultured at 30 °C with continuous shaking at 200 r.p.m. in iron-limited media (1 g L−1 NH4Cl, 2 g L−1 casamino acids, 28 g L−1 Instant Ocean (Aquarium Systems Inc.), 0.6% v/v glycerol), supplemented with 36 μM FeSO4 when required. PCR targeting (Gust et al., 2003; Eustáquio et al.