ExpI and PhzI Are Descendants of the Long Lost Cognate Signal Synthase for SdiA

SdiA of E. coli and Salmonella is a LuxR homolog that detects N -acyl homoserine lactones (AHLs). Most LuxR homologs function together with a cognate AHL synthase (a LuxI homolog), but SdiA does not. Instead, SdiA detects AHLs produced by other bacterial species. In this report, we performed a phylogenetic analysis of SdiA. The results suggest that one branch of the Enterobacteriaceae obtained a rhlR / rhlI pair by horizontal transfer. The Erwinia and Pantoea branches still contain the complete pair where it is known as expR / expI and phzR / phzI , respectively. A deletion event removed the luxI homolog from the remainder of the group, leaving just the luxR homolog known as sdiA . Thus ExpR and PhzR are SdiA orthologs and ExpI and PhzI are descendants of the long lost cognate signal synthase of SdiA.