Genomic and Functional Analysis of ICE Pda Spa1, a Fish-Pathogen-Derived SXT-Related Integrating Conjugative Element That Can Mobilize a Virulence Plasmid

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Abstract
Integrating conjugative elements (ICEs) are self-transmissible mobile elements that transfer between bacteria via conjugation and integrate into the host chromosome. SXT and related ICEs became prevalent in Asian Vibrio cholerae populations in the 1990s and play an important role in the dissemination of antibiotic resistance genes in V. cholerae . Here, we carried out genomic and functional analyses of ICE Pda Spa1, an SXT-related ICE derived from a Spanish isolate of Photobacterium damselae subsp. piscicida , the causative agent of fish pasteurellosis. The ∼102-kb DNA sequence of ICE Pda Spa1 shows nearly 97% DNA sequence identity to SXT in genes that encode essential ICE functions, including integration and excision, conjugal transfer, and regulation. However, ∼25 kb of ICE Pda Spa1 DNA, including a tetracycline resistance locus, is not present in SXT. Most ICE Pda Spa1-specific DNA is inserted at loci where other SXT-related ICEs harbor element-specific DNA. ICE Pda Spa1 excises itself from the chromosome and is transmissible to other Photobacterium strains, as well as to Escherichia coli , in which it integrates into prfC . Interestingly, the P. damselae virulence plasmid pPHDP10 could be mobilized from E. coli in an ICE Pda Spa1-dependent fashion via the formation of a cointegrate between pPHDP10 and ICE Pda Spa1. pPHDP10-Cm integrated into ICE Pda Spa1 in a non-site-specific fashion independently of RecA. The ICE Pda Spa1::pPHDP10 cointegrates were stable, and markers from both elements became transmissible at frequencies similar to those observed for the transfer of ICE Pda Spa1 alone. Our findings reveal the plasticity of ICE genomes and demonstrate that ICEs can enable virulence gene transfer.