Directing the Biosynthesis of Putrebactin or Desferrioxamine B in Shewanella putrefaciens through the Upstream Inhibition of Ornithine Decarboxylase

To manage iron acquisition in an oxic environment, Shewanella putrefaciens produces the macrocyclic dihydroxamic acid putrebactin (PB) as its native siderophore. In this work, we have established the siderophore profile of S. putrefaciens in cultures augmented with the native PB precursor putrescine and in putrescine-depleted cultures. Compared to base medium, PB increased by two-fold in cultures of S. putrefaciens with 10 mM NaCl and 20 mM exogenous putrescine. In cultures augmented with 1,4-diaminobutan-2-one (DAB), PB decreased with only 0.02-fold PB detectable at 10 mM DAB. As an ornithine decarboxylase (ODC) inhibitor, DAB depleted levels of endogenous putrescine which attenuated downstream PB assembly. Under putrescine-depleted conditions, S. putrefaciens produced as its replacement siderophore the cadaverine-based desferrioxamine B (DFO-B), as characterised by ESI-MS of the Fe(III)-loaded form (m/z(obs) 614.13; m/z(calc) 614.27). A third siderophore, independent of DAB, was observed in low levels. LC/MS Analysis of the Fe(III)-loaded extract gave m/z(obs) 440.93, which, formulated as a 1:1 Fe(III) complex with a macrocyclic dihydroxamic acid, comprising one putrescine- and one cadaverine-based precursor (m/z(calc) 440.14). These results show that the production of native PB or non-native DFO-B by S. putrefaciens can be directed though upstream inhibition of ODC. This approach could be used to increase the molecular diversity of siderophores produced by S. putrefaciens and to map alternative diamine-dependent metabolites.