Escherichia coli K-12 AB1157 (${\rm rec}^{+}{\rm uvr}^{+}$), AB2500 (uvrA), AB2470 (recB), and JC1553 (recA), which have different DNA repair capacity, were bombarded with α particles, C ions, and N ions having various beam energies in order to determine the LET-dependent radiosensitivity. Radiosensitivity of the uvrA mutant was almost the same as that of the wild-type throughout the whole LET region studied. Difference in radiosensitivity between the${\rm rec}^{-}$ mutants and the wild-type was found to decrease with increasing LET of charged particles used. Such LET dependence can be explained by the increase in the ratio of irreparable to reparable-type damages in DNA with the increasing LET of the particles. It is also possible that a significant component of the LET-dependent changes in viability observed may be due to damages to the repair system itself.

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