Mycobacteriophages D4, D29, D29A and D32, their isolated DNAs and their host, , 607, are highly resistant to u.v. irradiation. Infective DNA appeared more resistant than intact phage but this difference was in part due to shielding by the greater u.v. absorbency of the DNA solutions. It was, however, also due to the occurrence of non-reversible u.v. damage to phage protein. D29, D29A and D32 and their DNAs showed two-component dose-survival curves, D4 and its DNA were inactivated exponentially, while showed a non-linear semilog curve with a distinct shoulder in the low-dose region. possesses a mechanism for reversing u.v. damage to the bacterial genome and to the genome of infecting phage by photoreactivation. Specific dark-repair mechanisms were not identified, except in the case of a significant level of acriflavin-sensitive host-cell reactivation demonstrated for D29. The observed enhancement of survival of heavily irradiated D29 and D4 by pretreatment of the assay bacteria with iodoacetate was interpreted as evidence that a delay of growth and division of the host, and/or a delay of phage replication, contributed indirectly to a dark-repair mechanism.


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