Involvement of pnp in survival of UV radiation in Escherichia coli K-12.

Polynucleotide phosphorylase (PNPase), a multifunctional protein, is a 3'→5' exoribonuclease or exoDNase in the presence of inorganic phosphate (P(i)), and extends a 3'-OH of RNA or ssDNA in the presence of ADP or dADP. In Escherichia coli, PNPase is known to protect against H(2)O(2)- and mitomycin C-induced damage. Recent reports show that Bacillus subtilis PNPase is required for repair of H(2)O(2)-induced double-strand breaks. Here we show that absence of PNPase makes E. coli cells sensitive to UV, indicating that PNPase has a role in survival of UV radiation damage. Analyses of various DNA repair pathways show that in the absence of nucleotide excision repair, survival of UV radiation depends critically on PNPase function. Consequently, uvrA pnp, uvrB pnp and uvrC pnp strains show hypersensitivity to UV radiation. Whereas the pnp mutation is non-epistatic to recJ, recQ and recG mutations with respect to the UV-sensitivity phenotype, it is epistatic to uvrD, recB and ruvA mutations, implicating it in the recombinational repair process.