Abstract
The effect of sublethal fluences (50-200 kJ m-2) of UV-A radiation (320-400 nm) in bacterial cells is a transient growth inhibition related to photo-modified tRNA and is associated with changes in membrane structure and function. Higher UV-A fluences result in cell death due to the production of reactive oxygen species, so far undetected at sublethal doses. Oxidative mechanisms of toxicity induced by 120 kJ m-2 UV-A radiation can be recorded by ultra-weak chemiluminescence, useful in quantifying oxidative reactions. When Escherichia coli was exposed to UV-A stress at a fluence rate equivalent to that of the Sun in the biosphere (33 W m-2), chemiluminescence levels were proportional to the photodamage. Chemiluminescence and photo-damage are linearly proportional and dependent on environmental conditions of the cells. It is postulated that in addition to tRNA photo-modification, UV-A alters the membrane structure of E. coli by oxidative damage, since changes in the membrane structure under different environmental conditions play a key role in the cell's response to UV-A injury.
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