| Summary: | Treatment of bacteriophage lambda lysogens of Escherichia coli with various physical and chemical agents leads to induction. This thesis reports some investigations into the mechanism of induction, in particular following UV irradiation. Lysogens of different ages were found to vary in the efficiency with which they are induced by small doses of UV irradiation, being least inducible at division and most inducible at a time which coincides approximately with the time when rounds of DNA replication are thought to be terminated. One can indirectly induce a ? prophage by introducing into a lysogen an entire irradiated plasmid. Experiments are described showing that a ?N- mutant is unable to indirectly induce a prophage when irradiated. This suggests that the fortuitous acquisition by mutation of the ability to be maintained semi-stably in the cell in a non-integrated state is not sufficient to give a replicon the ability, when irradiated, to indirectly induce. Evidence is also presented in support of the correlation which exists between treatments leading to induction and those leading to the inhibition of cell division. These results have been considered in terms of three general hypotheses. (1) Induction is caused by the accumulation of some component involved in the initiation of rounds of DNA replication. This is clearly inconsistent with data showing the age at which lysogens are most readily inducible. (2) Induction is related to the activity of some DNA repair system. This hypothesis is consistent with much of the data presented in this thesis but is difficult to reconcile in its simplest form with the observation that only certain types of irradiated DNA indirectly induce. (3) Induction is related to the inhibition of cell division. This hypothesis is consistent with published work and receives support from data presented in this thesis.
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