Figure 1.

Proposed induction pathway for the UV-inducible cell-sensitising function of ICE R391. Stimulation of RecA to its active form (RecA*) by UV irradiation results in the cleavage of the putative orfs90/91 repressor protein (orf96) allowing the transcription of orfs90/91 which putatively encode a transcriptional enhancer complex that activates/increases expression of the orf43 gene product as well as the previously documented UV-inducible orf4 (jef) [14]. Expression of orf43 is then cytotoxic to E. coli host cells. Evidence to support this hypothesised pathway includes: RecA has been well documented to be stimulated to its active form (RecA*) by single-stranded DNA generated from exposure to UV irradiation [16], the observation that the cell-sensitising function of ICE R391 requires the presence of recA in the host genome [6], the deletion of orf96 encoding a putative repressor protein cannot be achieved without the previous deletion of orfs90/91[8], and orfs90/91 have previously been documented to enhance the transcription of other ICE R391 genes after host cell exposure to UV irradiation, specifically orf4 (jef), proposed to promote element excision from the host genome [14]. Additionally the ICE SXT homologs setR (orf96) and setC/D (orfs90/91) have been documented to have a similar recA-dependent, stress-inducible relationship [17].