ICE R391, a prototype member of the SXT/R391 family of site-specific integrative conjugative elements (ICEs), frequently isolated from enterobacterial pathogens, exhibits an unusual, recA-dependent, UV-inducible, cell-sensitising function. This significantly decreases postirradiation cell survival rates in Escherichia coli host cells, a trait that would at first appear to be counterproductive in terms of adaptation to stress conditions. Construction and screening of a complete ICE R391 deletion library in E. coli identified three ICE R391 genes, orfs90/91, encoding a putative transcriptional enhancer, and orf43, encoding a putative type IV secretion system outer membrane-associated conjugative transfer protein, in the cell-sensitising function. Cloning and complementation of these genes confirmed their involvement in UV sensitising. Expression of both orfs90/91 and orf43 in wild-type E. coli indicated that orf43 encodes a cytotoxic gene product upon up-regulation. Deletion of the orf43 homologue in SXT, s050, also abolished its associated UV sensitisation. We hypothesise that ICE R391 and other members of the SXT/R391 family display decreased survival rates upon exposure to UV irradiation through the induction of orf43.