Sequences and map positions of the 'essential' genes whose products have the same functions of most T-even phages are similar. In contrast, genomes of different members of the family have different 'nonessential' genes interspersed between these essential genes (Fig. 1). The heterologies contribute to apparent exclusions of alleles of one phage by another, and to the species barriers between different members of the family. They first became evident as insertion or substitution loops in electron micrographs of heteroduplex DNA prepared in vitro by annealing single strands of T2, T4 and T6 DNA, and have been confirmed by sequence comparisons in many cases.

In some cases the sequence divergence reflects gene amplifications and permutations of duplicated sequences. The tail fiber genes of different T-even phages appear to have diverged by illegitimate recombinations with genes of other phages, including prophages residing in the host genome. Substitutions of sequence blocks of individual genes by foreign sequences can account for the variability between different members of the family. In turn, these substitutions allow adsorption to different hosts with different receptors, accounting for the remarkable coevolution of viral and host genomes.

Such illegitimate recombinations are not limited to genes for recognition proteins. Illegitimate pairing of partially homologous sequences and join-copy and join-cut-copy recombination (discussed above) were apparently involved in horizontal gene transfer of nonessential genes adjacent to the essential dCTPase gene, and probably other genes as well. Although inactivating their functions has little or no consequences for phage development in the laboratory, we surmise that these genes are important for viral growth and survival under different physiological conditions, in different hosts with different receptors or containing different prophages, and in the face of various restriction systems imposed by different hosts.

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