Functional interactions beween the DNA ligase of Escherichia coli and components of the DNA metabolic apparatus of T4 bacteriophage.

T4 phage completely defective in both gene 30 (DNA ligase) and the rII gene (function unknown) require at least normal levels of host-derived DNA ligase (E. coli lig gene) for growth. Viable E. coli mutant strains that harbor less than 5% of the wild-type level of bacterial ligase do not support growth of T4 doubly defective in genes 30 and rII (T4 30- rII- mutants). We describe here two classes of secondary phage mutations that permit the growth of T4 30- rII- phage on ligase-defective hosts. One class mapped in T4 gene su30 (Krylov 1972) and improved T4 30- rII- phage growth on all E. coli strains, but to varying degrees that depended on levels of residual host ligase. Another class mapped in T4 gene 32 (helix-destabilizing protein) and improved growth specifically on a host carrying the lig2 mutation, but not on a host carrying another lig- lesion (lig4). Two conclusions are drawn from the work: (1) the role of DNA ligase in essential DNA metabolic processes in T4-infected E. coli is catalytic rather than stoichiometric, and (2) the E. coli DNA ligase is capable of specific functional interactions with components of the T4 DNA replication and/or repair apparatus.