Examination of the cell sensitizing gene orf43 of ICE R391 suggests a role in ICE transfer enhancement to recipient cells.

SXT/R391 family of ICEs have been found to express an unusual function that enhances bacterial cell death post-UV irradiation. Previous analysis of ICE R391 found four core SXT/R391 ICE genes to be involved—orf96, orf90, orf91 and orf43. These genes functioned as part of a UV-inducible pathway, where upon exposure to UV, the levels of the Orf43 protein, a TraV homolog which we propose naming TraV(R391), were upregulated, resulting in increased cell sensitization. Here, we examined the effect of orf43 overexpression and found it led to host cell permeabilization. The inducing agent for orf43, UV irradiation, is also known to increase the ICE R391 extrachromosomal form and apparent conjugative transfer rate. We demonstrated, via conjugative transfer deficient mutants, that orf43 overexpression alone restored a small level of ICE R391 transfer to recipient cells via an unknown mechanism other than conjugation. TraV homologs have been reported to function in conjugative transfer. However, TraV(R391) is the first homolog to cause UV-associated cell sensitization. TraV(R391) when overexpressed must contain a unique adaptation or function which results in cell lysis and decreased survival. A hypothesis for retaining such a detrimental effect may be in its role of enhancing ICE survival upon cell damage.