Indeed, based on bioinformatic homology, orf43 is predicted to encode a putative TraV homolog, an outer membrane protein involved in the ICE type IV secretion system and thought to function in the construction and stabilisation of the outer-membrane portion of the mating pore required for ICE transfer by conjugation . Deletion of the ICE R391-encoded orf43 was recently shown to abolish the UV-inducible sensitising effect of this ICE while clones expressing orf43 under arabinose control were shown to compliment for the transfer deficiency but additionally mimic the cell toxicity associated with UV ALK inhibition induction . 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) . 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 , the observation that the cell-sensitising function of ICE R391 requires the presence of recA in the host genome , the deletion of orf96 encoding a putative repressor protein cannot be achieved without the previous deletion of GW572016 orfs90/91, and orfs90/91 have previously been documented to Cell Cycle inhibitor 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 . Additionally the ICE SXT homologs setR (orf96) and setC/D (orfs90/91) have been documented to have a similar recA-dependent, stress-inducible relationship . Here, a model is proposed (Figure 1) for the control of this unusual ICE R391 UV-inducible sensitising effect based on expression 3-oxoacyl-(acyl-carrier-protein) reductase data examining the key genes involved and supported by a number of directed ICE R391 deletions. Results and discussion orfs90/91 stimulate orf43 transcription after exposure to UV irradiation We previously demonstrated that over-expression of orf43 when cloned into the arabinose inducible pBAD33-orf43 construct was responsible for the UV-inducible sensitisation observed in ICE R391 and other ICEs of the SXT/R391 family . Mutagenesis data also suggested that the putative transcriptional controller encoded by orfs90/91 was also involved, although not directly. To investigate the relationship between orfs90/91 and orf43, we utilised both qualitative and quantitative RT-PCR targeting these genes in different mutant backgrounds and with and without UV irradiation.