The Type III Secretion System plasmid pPHDPT3 of Photobacterium damselae subsp. piscicida is stable in Australian isolates due to conserved non-repetitive g

Abstract

Photobacterium damselae subsp. piscicida (Pdp) is a host-adapted primary pathogen impacting finfish aquaculture worldwide, whose virulence evolution is driven by the mobilome. The pPHDPT3 plasmid encoding a type III secretion system is critical for Pdp virulence but unstable in vitro in European and Japanese isolates. Here we show that a stable ancestral variant is conserved in Australian isolates. The elusive pPHDPT3 variant has undergone gene loss and accumulated a ∼5.5 kb quadruple direct repeat, which could explain plasmid loss via the classic dimer catastrophe scenario. In addition, we hypothesise that a 189-aa serine recombinase encoded within this repeat, and also on pPHDP10 plasmid, may act as a plasmid resolvase, with its frequent loss further exacerbating the dimer catastrophe.