Abstract
In this study, we determined the function of a novel non-ribosomal peptide synthetase (NRPS) system carried by a streptococcal integrative conjugative element (ICE), ICESe2. The NRPS shares similarity with the yersiniabactin system found in the high-pathogenicity island of Yersinia sp. and is the first of its kind to be identified in streptococci. We named the NRPS product ‘equibactin’ and genes of this locus eqbA–N. ICESe2, although absolutely conserved in Streptococcus equi, the causative agent of equine strangles, was absent from all strains of the closely related opportunistic pathogen Streptococcus zooepidemicus. Binding of EqbA, a DtxR-like regulator, to the eqbB promoter was increased in the presence of cations. Deletion of eqbA resulted in a small-colony phenotype. Further deletion of the irp2 homologue eqbE, or the genes eqbH, eqbI and eqbJ encoding a putative ABC transporter, or addition of the iron chelator nitrilotriacetate, reversed this phenotype, implicating iron toxicity. Quantification of 55Fe accumulation and sensitivity to streptonigrin suggested that equibactin is secreted by S. equi and that the eqbH, eqbI and eqbJ genes are required for its associated iron import. In agreement with a structure-based model of equibactin synthesis, supplementation of chemically defined media with salicylate was required for equibactin production.
Highlights
Horizontal gene transfer has facilitated greatly microbial evolution and rapid adaptation to new niches
We determined the function of a novel non-ribosomal peptide synthetase (NRPS) system carried by a streptococcal integrative conjugative element (ICE), ICESe2
The NRPS shares similarity with the yersiniabactin system found in the highpathogenicity island of Yersinia sp. and is the first of its kind to be identified in streptococci
Summary
Horizontal gene transfer has facilitated greatly microbial evolution and rapid adaptation to new niches. ICEs excise from the donor chromosome to form a non-replicative circular molecule, promote self-transfer to a new host by conjugation and subsequently insert into a new target site. This transfer can occur between genetically unrelated bacteria. Comparisons between ICEs of low G+C Gram-positive bacteria have revealed that these elements are widespread and possess a modular organization (Garnier et al, 2000; Roberts et al, 2001; Burrus et al, 2002b) Each of these modules includes all of the coding sequences required for a specific biological function, namely site-specific recombination, conjugation and regulation. A diverse set of mosaic ICEs are emerging; most encode a tyrosine recombinase (like the majority of integrated prophage) but a small number have been identified that utilize a serine recombinase to mediate site-specific recombination (Wang et al, 2000; Burrus et al, 2002a)
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