Fig 1. ICEMcSym1271 assembly, excision and regulation.
(A) Schematic of the possible ICEMcSym1271 recombination states and recombination reactions leading to formation of excised ICEMcSym1271 assembled from regions α, β and γ. The recombination targets attP, attB, attL and attR (triangles) and recombination reactions (large arrows) are color-coded for each integrase: cyan for IntS; magenta for IntG; green for IntM. Primers for qPCR measurement of recombination are indicated as block-headed arrows for attB sites and triangle-headed arrows for attP sites (see Materials and Methods and S1 Table for details). Data presented here support a model where RDF-stimulated excisive reactions occur in the order IntS > IntG > IntM (highlighted in yellow) to produce excised ICEMcSym1271. (B) The regulatory model of quorum-sensing mediated stimulation of ICEMcSym1271 assembly and excision. TraR1 and TraR2 bind AHLs produced by TraI1. TraR1/2-AHL complex(es) activate transcription from the traI1 and traI2 promoters. traI2-msi172-msi171 expression leads to production of FseA and transcriptional activation of the rdfS operon. RdfS stimulates excisive IntS-mediated recombination and promotes expression of RdfG and RdfM. RdfG stimulates the excisive IntG-mediated reaction and RdfM stimulates excisive IntM-mediated recombination and excision.