Comparison of the physical maps and redundant ends of the chromosomes of phages 2C, SP01, SP82 and phi e.
The physical map of 2C DNA (cf. following paper in this journal) was compared to the maps of SP01, SP82 and phi e (three other Bacillus subtilis phages containing hydroxymethyluracil in place of thymine in their DNA). The overall organization of the four genomes was remarkably similar, as indicated by the topology of HaeIII and SalI cleavage segments. The proof was gathered for the presence in the four phage DNAs of large redundant ends carrying a single HaeIII recognition site. The location of the latter proved identical for 2C and SP01, but was shifted in the DNAs of SP82 and phi e. Since the redundant end components of these hydroxymethyluracil genomes are colinear, as shown by cross-hybridization studies, the shifting of the HaeIII cleavage site is presumably due to two base substitutions, suppressing an endonuclease recognition site and establishing a new site elsewhere. Relatedness between the genomes of this family of viruses was evaluated from the fraction of conserved restriction fragments. According to these calculations, 6% base substitutions have occurred within the four viral DNAs, in the course of evolution. However, specific segments of 2C DNA were not present in SP01 and phi e DNA, as shown by cross-hybridization with restriction fragments. These data indicate the occurrence of deletions, in addition to base substitutions, as evolutionary mechanisms prevailing in the genomes of this family of phages.