The presence of inorganic or organic iron in the growth medium induces the production of a reddish-violet pigment in This pigment protects the organism against short exposures to ultraviolet (u.v.) radiation of wavelength below 300 mμ. The presence of iron also increases the degree of recovery of the normally white organism after such irradiation. Anaerobic growth renders both the red and the white micrococci more resistant to damage by u.v. radiation. However, the addition of nitrate, an alternative to oxygen as a terminal electron acceptor, restores the u.v. sensitivity of these micrococci to that of aerobically grown organisms. It is suggested that the initial site of u.v. irradiation damage is extranuclear and involves one or more cytochrome-linked systems, and that it is within this system(s) that the iron functions both as a radiation-protective and as a restorative agent.


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