DNA Synthesis and Gene Expression in Bacillus subtilis Infected with Wild-Type and Hypermodification-Defective Bacteriophage SP10

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A hypermodified base (Y-Thy) replaces 20% of the thymine (Thy) in mature DNA of Bacillus subtilis phage SP10. Two noncomplementing hypermodification-defective (hmd) mutants are described. At 30°C, hmd phage carried out a normal program, but at temperatures of ≥37°C, the infection process was nonproductive. When cells were infected at 37°C with hmd phage, DNA synthesis started at its usual time (12 min), proceeded at about half the normal rate for 6 to 8 min, and then stopped or declined manyfold. All, or nearly all, of the DNA made under hmd conditions consisted of fully hypermodified parental DNA strands H-bonded to unhypermodified nascent strands. The reduced levels of DNA synthesis observed under hmd conditions were accompanied by weak expression of late genes. A sucrose gradient analysis of SP10 hmd+ replicating DNA intermediates was made. Two intermediates, called VG and F, were identified. VF consisted of condensed DNA complexed to protein; VF also contained negatively supercoiled domains covalently joined to relaxed regions. F was composed of linear concatenates from which mature DNA was cleaved. None of those intermediates was evident in cells infected at 37°C with hmd phage. Shiftup experiments were performed wherein cells infected with hmd phage at 30°C were shifted to 37°C at a time when replication was well under way. DNA synthesis stopped or declined manyfold 10 min after shiftup. The hmd DNA made after shiftup was conserved as a form sedimentationally equivalent to the F intermediate, but little mature DNA was evident. It is proposed that Y-Thy is required for replication and DNA maturation because certain key proteins involved with these processes interact preferentially with hypermodified DNA.

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