Compounds of the cysteine-cysteamine group and their influence on infectivity, strand breaks and base damage in gamma-irradiated DNA of coliphage phiX-174.

Singleand double-stranded (RF) DNA of page <?X-174 were 60Co-gamma irradiated. Depen­ dence of radiosensitivity on the concentration of cysteamine and cystamine was measured. Radio­ resistance of DNA in vivo (phage particles) as well as of DNA in vitro (infectious DNA) increase between 10-4 and 10—1 M of superprotective agents. The same relationship is observed when the DNA is in the dry and in the wet state. By fractionating radiation damaged RFI and radiation induced RFII by means of sucrose gradient sedimentation it was possible to measure the protective effect of cysteamine on lesions other than strand-breaks. A pronounced radioprotective effect of cysteamine on the production of single-strand breaks in RF —DNA was found (DRF 20). The con­ venience of the RF system for studying effects of radioprotective agents on the occurrence of dif­ ferent radiation induced lesions is discussed.