Abstract— Near‐ultraviolet (near‐UV; 320–405 nm) irradiation of Escherichia coli B/r induces the formation in vivo of 4Srd‐Cyd adducts in transfer RNA, as evidenced by (1) fluorescence spectrum changes of tRNA extracted from irradiated cells and reduced with NaBH4, (2) thin‐layer chromatography on cellulose of hydrolysates of trichloroacetic acid‐precipitable extracts of irradiated cells, and (3) comparison of these findings with adduct formation induced by near‐UV irradiation of purified mixed tRNA from E. coli. The kinetics of induction of the 4Srd‐Cyd adduct in vivo, and the near‐UV fluences required, provide strong support for our earlier hypothesis that formation of these adducts is responsible for near‐UV‐induced growth delay in E. coli.

A mutant of Escherichia coli has been isolated that lacks 4-thiouridine, a rare base in transfer ribonucleic acid. The mutant grows at the same rate as wild-type cells. It shows little near-ultraviolet-induced growth delay, thus supporting earlier hypotheses that 4-thiouridine in transfer ribonucleic acid is the chromophore for this growth delay.

Abstract— Experiments on cell survival in concentrated suspensions of Escherichia coli B/r show that application of the well known “Morowitz correction” (H. J. Morowitz, (1950) Science111, 229–230) can lead to large errors in estimation of the average fluence per cell if light scattering is not taken into account. The magnitude of the effect is illustrated for this organism, but it is pointed out that experimenters should determine the correction for each organism and set of experimental conditions used.

Near-ultraviolet (near-UV) light (300 to 380 nm) is a significant component of sunlight and has a variety of effects on biological systems. The present work is an attempt to identify chromophores (molecular absorbers of light) and targets (critical damaged molecules) for inhibition of adenosine triphosphate (ATP) synthesis in Escherichia coli by near UV. The fluence of 334 nm required for 37% survival of net ATP synthesis (F37) in E. coli AB2463 in succinate medium is 140 kJ/m2. The action spectrum for this inactivation is almost structureless, exhibiting a smooth transition from high efficiency at 313 nm to low efficiency at 405 nm. The action spectrum for inhibition of net ATP synthesis is consistent with the chromophore being either ubiquinone Q-8 or vitamin K2. The fluence required is consistent with ubiquinone Q-8 also being a target molecule. The activity of reduced nicotinamide adenine dinucleotide dehydrogenase in extracts of E. coli B is also inactivated by near UV and shows an F37 of about 40 kJ/m2. The action spectrum for this effect is quite structureless; it shows high efficiency at 313 nm and low efficiency at 435 nm. The data do not suggest a target molecule for this action, although it is possible that ubiquinone Q-8 absorbs the near-UV energy and then passes it on to some other target molecule. The data further indicate that inactivation of the oxidative phosphorylation system is not a primary factor in near-UV-induced growth delay in E. coli.

Abstract— Near‐ultraviolet radiation (near UV; 300–380 nm) has long been known to produce a transient reduction of the capacity of bacteria to support phage growth. The present work shows that, at high fluenœs (40–100 kJ/m2), 85% of 334‐nm‐induced reduction of capacity in Escherichia coli B/r requires the rel gene; that is, it results from rel‐gene activity caused by the near‐UV treatment. This rel‐gene activity leads to (1) a bacterial growth delay and concomitantly lowered bacterial metabolism, and (2) a parallel delay in phage development, with a considerable depression of burst size. We propose that the observed effects on phage development are a consequence primarily of the lowered bacterial metabolism, but they may also result partly from a direct inhibition of phage DNA synthesis by the rel gene product, these effects together leading to the observed reduction of capacity in a rel+ strain. The remaining 15% of capacity reduction, observed in a rel strain, has an unknown mechanism, but does appear to involve a delay in phage development. At least 95% of the total capacity reduction observed in the rel+ strain in the range 40–100 kJ/m2 requires the presence of 4‐thiouridine, an unusual base in E. coli transfer RNA, which is presumably both the chromophore and the target for near‐UV‐induced capacity reduction.