Replication cycle of Bacillus subtilis hydroxymethyluracil-containing phages.

The present review focuses on phage 2C, a member of a family of virulent phages that multiply in Bacillus subtilis. The best known members of this group are SPO1, phi e, H1, 2C, SP8, and SP82, the genomes of which are made of double-stranded DNA of about 150 kilobase pairs (kbp). The two DNA strands have different buoyant densities. Moreover, thymine (T) is completely replaced by hydroxymethyluracil (hmUra). Comparison of the phage DNAs has shown that both base substitutions and deletions have contributed to the evolution of their genomes. In addition, all of the hmUra-phage genomes contain colinear redundant ends, amounting to 10% of total bases. Two lines of evidence suggest that the redundant ends of 2C DNA, in spite of extensive homology, contain unique sequences. Further studies focused on DNA replication during the lytic cycle. The semiconservative replication of the infecting viral genome is followed by extensive recombination. At the level of replication forks, viral DNA synthesis is discontinuous on both strands during the whole cycle. Deoxythymidinetriphosphate, required for viral DNA synthesis in permeabilized infected bacteria, was incorporated in small amounts into phage DNA. The putative primary origin of replication has been cloned and localized on the viral genome. Some viral promoters have been successfully cloned in Escherichia coli. These sequences, however, did not promote transcription in B. subtilis. The abnormal base might be required for promoter activity in the natural host.