Symmetrical nucleotide sequences in the recognition sites for the ter function of bacteriophages P2, 299 and 186

The 3′-terminal sequences of DNA from the temperate coliphages P2, 299 and 186 have been determined from analyses of radioactive oligonucleotides in DNAase digests of DNA preparations that had been labelled at their 3′ termini. Together with the sequences previously determined for the 5′ cohesive ends of DNA from these phages, the results define a sequence of 33 base-pairs in the region of the ends of these molecules. If these sequences are written with the cohesive ends of the DNA base-paired, they form a duplex region called cos. The cos sequences for the three DNAs are very similar, but not identical, and an axis of 2-fold rotational symmetry can be drawn through their central pair of bases. In each case, two blocks of sequences flanking the paired cohesive ends (i.e. at the 3′ termini of the mature DNA molecule) were symmetrically disposed about this axis, but most of the 19 base-pairs within the cohesive region were non-symmetrical. The symmetrical parts of these sequences may be involved in recognition of the cos site by the phage ter system, which is responsible for formation of the cohesive ends of the mature DNA molecules.