DNA repair mutational analysis

A strain of E.coli contains a uvr mutations and thus has a lower % survival when exposed to increasing levels of UV. An alternate strain X contains an unknown mutation that has an even lower % survival (even more so than uvr mutation independently). However when these two are put together they have much lower % survival. Experiements showed that mutating more uvr components had no effect that mutating just one. How would one know the identity of the unknown mutation AND make simple experiments to verify this?

I know that given additional uvr mutations means that the mutation cannot be an Nucleotide Excision Repair component mutation, thats about it?