Non-covalently bound adenine nucleotides in adenosine triphosphatase of Escherichia coli.

The term, xeroderma pigmentosum variants designates patients who suffer from the clinical manifestations of the disease, but whose cells have normal rates of excision repair of UV-induced lesions in DNA. In contrast to normal human fibroblasts, if cells from such variants are maintained in medium containing caffeine from immediately following exposure to UV until the survivors have undergone three doublings, the cytotoxic and mutagenic effect of UV light is dramatically increased. In the presence of 0.7mM caffeine, the slope of the UV survival curve increases $\text{ca.}$ 3-fold. Similarly, the slope of the curve describing the frequency of mutations to azaguanine resistance induced by UV as a function of dose is $\text{ca.}$ 3-fold steeper.