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Abstract— Non‐dividing human cells degenerate and eventually detach from a culture vessel surface when exposed to UV light. Action spectra for this kind of cell inactivation were determined using eight monochromatic wavelengths from 240 to 313 nm and both a normal DNA excision‐repair‐proficient strain and a repair‐deficient Xeroderma pigmentosum (XP12BE) strain. The action spectra for both strains have similar shapes with a broad peak between 254 and 280 nm followed by a steep decline at wavelengths greater than 280 nm. The relative action spectra are similar to those for inactivation of reproductive capacity and pyrimidine dimer formation in rodent cells suggesting that the critical target and critical damage for inactivation of non‐dividing human cells is DNA and damage to DNA, respectively. Normal repair‐proficient cells are 5–7 times more resistant at all wavelengths, based on a comparison of Do values, than repair‐deficient XP12BE cells, supporting the conclusion that the inactivating damage at all wavelengths is to DNA.

Nondividing human diploid fibroblasts maintained in medium containing 0.5% calf serum do not survive when exposed to low doses of UV (254 nm). The extent of killing is dose and strain dependent. DNA excision repair-proficient cells are more resistant than excision repair-deficient cells. Results of measurements of the effect of UV on RNA and protein synthesis in repair-proficient and -deficient (XP12BE) cells are reported. UV causes an immediate and equal depression of the RNA synthesis rate in both kinds of cells. A recovery to control rates was observed only at low (5 J/m2) doses in repair-deficient cells and at higher doses (20 J/m2) in repair-proficient cells. No recovery was observed at doses that cause substantial reductions in survival (greater than 5 J/m2 for XP12BE; greater than 40 J/m2 for repair-proficient populations). No initial effect on rate of protein synthesis was detected at doses less than 20 J/m2. However, in XP12BE populations, a decreased rate first evident at 15-30 h post-UV and before any cell degeneration and loss was observed for doses as low at 7 J/m2. This delayed effect was not observed in repair-proficient populations. The results are consistent with the hypothesis that the lethal action of UV in nondividing cells is one on DNA that leads to an inhibition of required protein synthesis by preventing RNA transcription.

Short periods of incubation in medium containing nalidixic acid or hydroxyurea, followed by a return to normal growth conditions, induced filament formation in Escherichia coli B (fil+) and AB1899NM (Ion-) but not in B/r (fil-) and AB1157 (lon+). These drugs reversibly stopped deoxyribonucleic acid (DNA) synthesis with little or no effect on ribonucleic acid (RNA) synthesis or mass increase. The initial imbalance caused by incubation in these drugs was the same for B and B/r as was macromolecular synthesis following a return to normal growth conditions. DNA degradation caused by nalidixic acid was measured and found to be the same for B and B/r. Hydroxyurea caused no DNA degradation in these two strains. Survival curves as determined under various conditions by colony formation suggested that the property of filament formation was responsible for the extrasensitivity of fil+ and lonstrains to either nalidixic acid or hydroxyurea. E. coli B was more sensitive to either drug than was B/r or B,-1. Pantoyl lactone or liquid holding treatment aided division and colony formation of nalidixic acid-treated B but had no effect on B/r. Likewise, the filament-former AB1899NM was more sensitive to nalidixic acid than was the non-filament-former ABI 157. The sensitivity of B/r and B.,1 to nalidixic acid was nearly the same except at longer times in nalidixic acid, when B8-1 appeared more resistant. Even though nalidixic acid, hydroxyurea, and ultraviolet light may produce quite different molecular alterations in E. coli, they all cause a metabolic imbalance resulting in a lowered ratio of DNA to RNA and protein. We propose that it is this imbalance per se rather than any specific primary chemical or photochemical alterations which leads to filament formation by some genetically susceptible bacterial strains such as lon-and fil+.Certain strains of Escherichia coli, referred to as MATERIALS AND METHODS Bacterial strains and growth conditions. The bacterial strains used in this investigation were E. coli B, 520 on July 10, 2020 by guest