A restriction map and analysis of the terminal redundancy in the group a streptococcal bacteriophage SP24.

The DNA isolated from the group A streptococcal bacteriophage SP24 is a linear double-stranded molecule 42.0 kb in length. The DNA has been characterized by electron microscopy, and by agarose gel electrophoresis after cleavage with the restriction endonucleases SalI, BglII, XbaI, PvuI, HindIII, and BamHI. Analysis of SalI digests indicates that two fragments are present in submolar amounts and exist as a subset of sequences present in another SalI fragment. Moreover, overlapping endonuclease fragments suggested that the physical map is circular. This was confirmed when homoduplex phage DNA revealed circular structures with single-stranded tails that were 7.7% of the circumference of the genome length molecule. Tails were observed to be separated by as much as 42% of the circular homoduplex structure. These results indicate that the phage SP24 genome is terminally redundant and circularly permuted; and the data are consistent with a model in which DNA packaging into phage heads is initiated at a specific site on concatermeric DNA and proceeds sequentially to package up to five "headfuls" of DNA per concatemer.