The effect of helper phages and/or multiplicity of infection on the repair of ultraviolet damages in T4

Abstract The problem raised by the ultraviolet (UV)-target sizes of T4 cistrons determined by the survival of phenotype (SP survival) of irradiated wild-type phage in mixed infection with unirradiated carrier ( Krieg, 1959 ) is investigated. It is noted that the greatly overestimated target sizes obtained by this latter method particularly for macrotarget functions ( Womack and Barricelli, 1965 ) are comparable to the mean target size of the three vulnerable centers determined by multiplicity reactivation ( Barricelli, 1956 ). A suggestion by Harm (1966) that the three vulnerable centers represent early functions required for repairing processes like V-gene reactivation, X-gene reactivation, and possibly other similar processes hereafter collectively designated as “spontaneous reactivation,” is used as a basis for the following interpretation: the greatly overestimated SP-target sizes are assumed to be the effect of a lack of or a reduction of spontaneous reactivation, when an irradiated phage competes with unirradiated phages for phenotypic expression. In order to test this interpretation, a series of SP-survival experiments has been done in which not only the wild-type phage, but also the carrier was irradiated. In each experiment only one UV dose to the carrier (usually smaller than all UV doses given to the wild-type phage) was used while the wild type received several UV doses in order to obtain an SP-survival diagram for irradiated carrier. A UV-dose to the carrier is expected to give a certain dose-dependent probability for spontaneous reactivation of damages in the wild-type cistron of the coinfecting phage. This is a necessary consequence of multiplicity reactivation theory ( Barricelli, 1956 , Barricelli, 1960 ) and the above interpretation ( Harm, 1966 ) implying that, when all phages are equally irradiated, spontaneous reactivation normally occurs in all cistrons except the 3 vulnerable centers. As a result the SP-survival values with irradiated carrier are expected to be significantly higher than one would expect if all damages in the wild-type cistron measured by the SP-survival method were phage lethal damages (instead of partly spontaneously reparable or “sublethal”). In fact it can be expected that a small UV-dose to the carrier may, under certain conditions, increase rather than reduce the SP-survival. All these predictions are confirmed by the experiments. Furthermore it has been possible to outline some of the rules specifying the conditions for spontaneous reactivation of the sublethal wild-type cistron damages, thus obtaining a fairly accurate interpretation of the experimental data. Although the outlined theory is still rudimentary it is already clear that UV-hits (sublethal as well as lethal) in the mutant cistron of the carrier phage are the main factor which makes possible spontaneous reactivation of sublethal damages in the wild-type cistron. This effect does not have to be due to a stimulation of the spontaneous reactivation processes, in a selective manner for the cistron involved. Simply by delaying or preventing the production of the mutant protein, damages in the mutant cistron may buy the time necessary for spontaneous reactivation of the wild-type cistron and production of wild-type protein in time to establish the wild phenotype. This would be possible if the mutant protein is not deprived of all function but is, on the contrary, able, although in a lethal or abnormal fashion, to carry the processes of infection to a stage in which the wild-type protein would no longer be effective. On the other hand, the absence of both proteins may interrupt or suspend the processes at an earlier stage so that the wild phenotype may be expressed as soon as the appropriate protein becomes available.