You know that Cytosine can spontaneously delaminate to Uracil in DNA. dCTP, a substrate for the synthesis of DNA, can as well spontaneously delaminate to dUTP. E. coli has an enzyme called Dut (dUTPase) whose job it is to get rid of any dUTP in cell. Mutations which cause loss of Dut are LETHAL in strain that is as well missing RecA. (In other terms, a dut- mutant is alive, a recA- mutant is alive, but the dut- recA- mutant is dead.) An additional mutation in gene encoding Ung, the Uracil N-glycosylase which removes Uracil from DNA, restores viability to the dut- recA- double mutant. (The dut- recA- ung- triple mutant is alive.)
What is mechanism of lethality in mutant that lacks Dut and RecA and why is this lethality suppressed by loss of the Ung?