UC Santa Barbara

Acyl peptidic siderophores are produced by a variety of bacteria and are often produced in a suite where the iron(III)-binding headgroup remains constant while the fatty acid appendage varies by length and functionality. The inclusion of a fatty acid tail is clearly advantageous for some siderophores, such as the mycobactins, whose amphiphilic properties allow the siderophore to freely diffuse between membranes and media for iron acquisition from macrophages; however, for the majority of acyl siderophores, the biological significance of the fatty acid tail has yet to be determined. This work is focused on the production and post-assembly modification of acyl siderophores.

The ubiquitous marine hydrocarbon-degrading bacterium, Alcanivorax borkumensis SK2, was determined to produce the amphibactins, which have been isolated from multiple oil-associating marine bacteria. Only longer tailed amphibactins (C16 and C18) were isolated from the cell pellet of A. borkumensis SK2 following centrifugation, as identified by tandem mass spectrometry. Putative biosynthetic genes for amphibactin biosynthesis were also evaluated in the genome of A. borkumensis SK2.

The marine bacteria Marinobacter sp. DS40M6 and Marinobacter nanhaiticus D15-8W produce a suite of acyl peptidic marinobactin siderophores to acquire iron under iron-limiting conditions. During late-log phase growth, the marinobactins are hydrolyzed to form the marinobactin headgroup with release of the corresponding fatty acid tail. The bntA gene, a homologue of the Pseudomonas aeruginosa pyoverdine acylase gene, pvdQ, was identified from Marinobacter sp. DS40M6. A bntA knockout mutant of Marinobacter sp. DS40M6 produced the suite of acyl marinobactins A-E, without the usual formation of the marinobactin headgroup. Another marinobactin-producing species, Marinobacter nanhaiticus D15-8W, is predicted to have two pvdQ homologues, mhtA and mhtB. MhtA catalyzes hydrolysis of the apo-marinobactin siderophores, as well as, the quorum sensing signaling molecule, dodecanoyl-homoserine lactone. In contrast to hydrolysis of the suite of apo-marinobactins by MhtA, hydrolysis of the iron(III)-bound marinobactins was not observed.

The post-assembly modification of another acyl siderophore is seen with the amphi-enterobactins produced by Vibrio harveyi BAA-1116. A dimer of 2,3-dihydroxybenzoyl-L-Ser is found in the cell-free supernatant of V. harveyi BAA-1116 following centrifugation. V. harveyi BAA-1116 does not possess biosynthetic genes for 2,3-dihydroxybenzoyl-L-Ser, suggesting the dimer is formed as a hydrolysis product of the amphi-enterobactins. Adding Fe(III)-amphi-enterobactins to crude cell-free extracts of V. harveyi BAA-1116 results in dimer formation; however, dimer formation is not seen when the apo-amphi-enterobactins are incubated with cell-free extracts, suggesting this could be a mechanism for iron release and siderophore recycling.