Inhibitory effect of agents altering the structure of DNA on the synthesis of pyrimidine deoxyribonucleotides in bacteriophage T4 DNA replication

Incorporation of thymine-containing DNA precursors in plasmolysed cells infected by the T4 non-lethal recombination defective mutants

Incorporation of TdR is aberrant in cells plasmolysed 15 min after infection by the recombination defective t4 chi and omega mutants. The in situ results parallel those obtained in vivo: at high TdR concentrations both T4 chi and T4 omega induced incorporation is slightly reduced compared to wild type, whereas at low TdR concentration incorporation induced by T4 chi is reduced and that induced by T4 omega is increased compared to wild type. No differences between wild type and mutant induced TdR incorporation are observed when cells are plasmolysed 8 min after infection. Further, no difference in incorporation between wild type and T4 chi or T4 omega is observed when either 3H thymine or 3H dTTP is used as a substrate, however small incorporation differences are observed using 3H dTMP as substrate. The mitomycin C sensitivity of T4 chi induced TdR incorporation is also observed in situ, but the drug must be present throughout infection. T4tk omega mutants have increased ability to incorporate 1 microM 3H TdR compared to T4tk and the reduced incorporation of 1 microM 3H TdR by T4 chi is suppressed in a T4td chi double mutant. These data are compatible with the hypothesis that endogenously produced TdR modulates leading and lagging strand synthesis and that the aberrant 1 microM TdR incorporation exhibited by T4 chi and T4 omega reflects specific activity changes resulting from a recombination defect induced alteration of the TdR "modulator pool".

Phenotypic differences among the alleles of the T4 recombination defective mutants

The allelic forms of the phage genes T4 chi and fdsA, as well as T4y and fdsB are compared in terms of their thymidine incorporation in high or low concentrations of thymidine, sensitivity of DNA synthetic capacity to mitomycin C, and sedimentation rates of DNA replicative intermediates. The results show differences among these mutants for the incorporation of thymidine; however all exhibit mitomycin C-sensitive DNA synthesis and have identical aberrant sedimentation rates for their DNA replicative intermediates.

Properties of the nonlethal recombinational repair deficient mutants of bacteriophage T4. III. DNA replicative intermediates and T4w

The rate at which 3H thymidine is incorporated into DNA is increased in T4w-infected cells compared to wild-type when measured late in infection under conditions of low thymidine concentration. This increased DNA synthesis is sensitive to hydroxyurea but not to mitomycin C, and can be prevented by the addition of chloramphenicol early in infection. Also, DNA replicative intermediates isolated from T4w-infected cells late in infection sediment significantly faster than those isolated from wild-type-infected cells. In contrast, DNA replicative intermediates isolated from T4x- or T4y-infected cells sediment more slowly than those produced by wild-type T4. Cells coinfected with wild-type T4+ and T4x, y or w; or T4w and T4x or y, produce wild-type DNA replicative intermediates. Cells coinfected with T4x and T4y produce more slowly sedimenting DNA replicative intermedites. Cells coinfected with T4w and wild-type T4 show wild-type rates of DNA synthesis while cells coinfected with T4w and T4x or T4y show increased rates of DNA synthesis over that observed with wild-type alone.