Abstract
BackgroundMany SXT/R391-like enterobacterial Integrative Conjugative Elements (ICEs) have been found to express an atypical, recA-dependent, UV-inducible, cell-sensitising phenotype observed as a reduction in post-irradiation cell survival rates in host cells. Characterisation of a complete deletion library of the prototype ICE R391 identified the involvement of three core ICE genes, orfs90/91 encoding a putative transcriptional enhancer complex, and orf43, encoding a putative type IV secretion system, outer membrane-associated, conjugative transfer protein.ResultsIn this study, expression analysis of orf43 indicated that it was up-regulated as a result of UV irradiation in an orfs90/91-dependent manner. Induced expression was found to be controlled from a site preceding the gene which required functional orfs90/91. Expression of orfs90/91 was in turn found to be regulated by orf96, a λ cI-like regulator. Targeted construction of ICE R391 deletions, RT-PCR and qRT-PCR analysis confirmed a regulatory link between orfs90/91 and orf43 while site-directed mutagenesis of orf43 suggested an association with the cell membrane was a prerequisite for the cytotoxic effect.ConclusionsBecause of the recA-dependence of the effect, we hypothesise that UV induction of RecA results in cleavage of the cI-like ICE-encoded repressor protein, the product of orf96. This in turn allows expression of the transcriptional enhancer complex encoded by orfs90/91, which we conclude stimulates transcription of orf43, whose product is directly responsible for the effect.
Highlights
Many SXT/R391-like enterobacterial Integrative Conjugative Elements (ICEs) have been found to express an atypical, recA-dependent, UV-inducible, cell-sensitising phenotype observed as a reduction in postirradiation cell survival rates in host cells
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 [8]
Mutational analysis of the ICE R391 determined that the core genes orfs90/91 and orf43 were required for expression of the cell-sensitising function [8] while bioinformatic analysis indicated that orf96 likely encodes a λ cI-like repressor similar to RecA substrates in other phage systems that are cleaved following SOS induction [9]
Summary
Many SXT/R391-like enterobacterial Integrative Conjugative Elements (ICEs) have been found to express an atypical, recA-dependent, UV-inducible, cell-sensitising phenotype observed as a reduction in postirradiation cell survival rates in host cells. ICEs are divided into families based on similarity between core genes ( the integrase gene) and the site of integration they utilise within host chromosomes. The SXT/R391 family share a highly similar integrase gene and integrate into the prfC gene of enterobacterial hosts [1,3]. In addition to encoding host beneficial traits such as antibiotic resistance determinants [3,4,5], many SXT/R391 family ICEs express an unusual cell-sensitising function [6,7,8]. Preliminary characterisation of the UV-inducible, cell-sensitising function of the prototype, ICE R391, determined the effect to. Induction of RecA (some 50 fold) following UV irradiation, results in cleavage of phage λ and phage λ -like cI repressors resulting in phage induction and cleavage of other SOS repressors [10,11,12,13]
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