Induction and Reversion of c-Mutations by Irradiation of the Extracellular ϰ-Phage of Serratia

Abstract

IT was reported earlier¹ that mutations to clear-plaque type are induced by ultra-violet irradiation of the ϰ-phage in the extra-cellular state (without irradiation of the host bacteria) up to about 2 per cent as a two-hit process. Such c-mutations appear also when the same phage is irradiated as prophage of the lysogenized Serratia strain HY (ϰ) (ref. 2). Since the rate of induced mutation in this intracellular prophage is about the same as in the free phage, ‘indirect’ effects of ultra-violet radiation via the intracellular environment do not seem to play an essential part in this mutation-induction process. Postcultures of single fresh mutant plaques show pure mutant type in most cases; therefore, both chains of the deoxyribonucleic acid seem frequently to be changed by the mutation induced by ultra-violet irradiation. A few postcultures gave a mixture of c and normal plaques, and the second postcultures of these c plaques were pure c. Thus, a kind of ‘heterozygous’ segregating state, probably due to change of only one base partner in the deoxyribonucleic acid double chain, seems to be caused by some of the mutating events.