Structures and broad-spectrum growth-inhibiting activity of formomarinobactin, formylated marinobactin analogues from the Pseudomonas lutea clade

Pseudomonas graminis LMG 21661T, an environmental strain of the P. lutea clade, produces the siderophore formomarinobactin, a novel marinobactin-like siderophore. Mass spectrometry revealed that formomarinobactin shares the same six-residue peptide backbone as marinobactin but contains formylated rather than acetylated N-hydroxyornithines. Alike marinobactins, formomarinobactins are produced as a suite of siderophores with a conserved hexapeptide core but varying lipid tail lengths (C10-C14), shorter than the C12 to C18 characteristic of marinobactins. Both the biosynthesis and cognate receptor genes of the formomarinobactin system in P. graminis are iron regulated but unaffected by zinc or nickel underscoring their role in iron homeostasis. Genome mining combined with mass analyses demonstrated that formomarinobactin production is a conserved trait across the P. lutea clade, with one exception which appears to represent an intraspecific cheater that has lost siderophore production. Beyond the producing strains themselves, we identified a widespread distribution of putative formomarinobactin receptors among diverse Pseudomonas species, revealing a substantial capacity within the P. fluorescens super clade to pirate formomarinobactin as an iron source. Putative receptors were also found in genera outside the Pseudomonas genus. Growth stimulation assays confirmed functional formomarinobactin uptake in several Pseudomonas spp. and a Phytopseudomonas strain, with genetic validation in Pseudomonas rhodesiae. Importantly, formomarinobactin production confers more than a nutritional advantage. Members of the P. lutea clade producing formomarinobactin display pronounced growth inhibiting activity against a broad spectrum of clinical and environmental Gram-positive and with lower efficacy against Gram-negative bacteria. Purified formomarinobactin was able to inhibit growth under iron-limiting conditions and to a lesser extent in iron-rich conditions, highlighting a dual role for this molecule in both iron acquisition and microbial growth inhibition.