DNA damage and mutagenesis of lambda phage induced by gamma-rays

Lambda phage DNA was gamma-irradiated in aqueous solution andthe amount of radiation-induced strand breakage [double- andsingle-strand breaks and alkali-labile sites (dsb, ssb, als)]was determined. Twice as much minor structural damage (ssb andals) per lethal hit was found in this DNA compared with DNAfrom irradiated phage suspensions. The in vitro irradiated DNAwas re-packaged into infectious particles (‘pack-phage’).The induction of mutations in the cI or cII cistron was scoredusing SOS-induced host cells. The in vitro prepared particleswere found to have second-order kinetics for mutagenesis inducedby gamma-rays indicating that two pre-mutational events werenecessary to produce a mutation. In contrast, bacteria-freephage suspensions (‘lys-phage’) showed single hitkinetics for mutagenesis after irradiation. The increase inthe mutation rate in the phage particles was mainly due to minorlesions, i.e. ssb, als and unidentified base damage. In lys-phage,mutagenesis might be enhanced by clustered DNA damage —a configuration which does not exist in pack-phage. The lossof infectivity was analysed in comparison with the extent ofstructural damage. All lesions investigated here contributedto biological inactivation. Minor lesions were tolerated bylambda phage to a limited extent. Major lesions, such as dsb,contributed most to the loss of infectivity and can be consideredas lethal events.