Problem of chromosome formation in T-even phages]

Three basic versions for the formation of circularly permuted and terminally redundant chromosomes with rings, concatemers, or fragments as replicative intermediates were considered. Experimental results show that the chromosome of T-even phage can turn into 4-6 large fragments soon after it penetrates inside the Escherichia coli cell. The fragments are capable for autonomous replication and contribute their material to progeny phage chromosomes. These results confirm the suggestion that circularly permuted and terminally redundant chromosomes of T-even phages are made of fragments. A theoretical analysis of different modes of parental chromosome fragments formation, autonomous replication and ordered association was carried out. In particular, it was emphasized that at a low multiplicity of infection the reassociation of fragments by means of recombination can be accomplished only if breaks in complementary strands of the parental chromosome were made with a shift for about 3000 nucleotides. Complexity is a feature of linear chromosomes that ensures their reproduction without defects at the ends.