The ends of mature T4 chromosomes are nearly randomly permuted over the map, and 3—5% of the genome is repeated at both ends as so-called terminal redundancies. Mature chromosomes are generated from the branched concatemeric vegetative DNA during packaging by a terminase, a heteromeric protein encoded by genes 16 and 17 that associates with DNA and with gp20 at the portal vertex of the head (Fig. 1) and uses ATPase activities for the head-filling process. Gene 17 produces several proteins of different sizes by initiation from in-frame internal initiation codons. Several of these proteins have nuclease activity; the largest one also binds non-specifically to single-stranded DNA segments. There is controversial evidence as to whether T4 has sequence specific pac sites or whether it initiates packaging at such random single-stranded DNA segments. Perhaps both processes can initiate packaging. If the first initiation of packaging is a relatively rare event, processive packaging of 103—105% genome lengths to fill the preformed heads can account for the random circular permutation of the ends in mature virion DNA by either mechanism. Endonu-clease VII which cuts Holliday and Y junctions and mismatched base pairs in vitro is required in vivo to trim the branches of vegetative DNA. It can associate with gp20 and retain nuclease DNA ligase, endo-nuclease V and topoisomerase are also required, presumably to provide uninterrupted double-stranded DNA as packaged chromosomes.
An in vitro packaging system has been developed to package large pieces of foreign DNA.
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