PHOTOREACTIVATION AND PHOTOPROTECTION*

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Abstract
Summary Survival after far ultraviolet irradiation can be greatly increased in most cells by a posttreatment with near ultraviolet or violet‐blue radiation (photoreactivation) or, in some cells, by a pretreatment with near ultraviolet radiation (photoprotection). In addition, many cells recover from some lethal far ultraviolet damage by posttreatments not involving light (dark recovery), and it seems likely that considerable recovery of this type occurs even under ‘normal’ postirradiation conditions that do not involve any special ‘treatments’. In this paper, three aspects of photoreactivation of lethal damage are considered: (1) the nature of photoreactivable damage, including the possibility of cytoplasmic and RNA, as well as DNA, damage; (2) the mechanism of direct (photoenzymatic) photoreactivation, with special reference to dose‐rate saturation and temperance dependence; and (3) relations among photoprotection, dark recovery, and photoreactivation, with particular reference to the phenomenon of ‘indirect photoreactivation’, which is similar in mechanism to photoprotection. The general approach is broad, an attempt being made to point out some gaps and inconsistencies in our knowledge of these phenomena.