Letter | Published:

Uncoupling of initiation site cleavage from subsequent headful cleavages in bacteriophage T1 DNA packaging

Nature volume 301, pages 264266 (20 January 1983) | Download Citation



The packaging of intracellular DNA into heads is a key feature in the morphogenesis of bacteriophage particles. In many phages a preformed empty head precursor, the prohead, is filled with DNA from a concatemeric substrate consisting of tandemly repeated genome lengths1,2. The addition of outer shell proteins completes head formation. The DNA molecules released from particles of the coliphage T1 exist as three major permutations of nucleotide sequence3,4. Such limited permutation can be explained by the modification of Streisinger's ‘headful’ mechanism proposed for phage P225,6. DNA packaging is initiated at a specific site (the pac site) on the concatemeric precursor. While this site is cleaved, subsequent cleavages (headful cleavages) are dependent only on head-filling and are not defined in terms of nucleotide sequence. Headfuls of DNA, consisting of slightly more than a genome length, are packaged in three successive cycles of head-filling to produce the permuted and terminally redundant molecules characteristic of T1 DNA. To elucidate the regulation of this process, we have studied the DNA metabolism of T1 head mutants. We describe here the properties of a mutant in gene 13.3 which is defective for headful cleavage but remains proficient in pac site cleavage. The observation in this mutant that concatemers are degraded to unit-length molecules by repeated pac site cleavage suggests a model of headful packaging in which pac site initiation and processive head-filling compete for the DNA substrate.

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    • N. Ramsey

    Present address: MRC Laboratory of Molecular Biology, Hills Road, Cambridge CB2 2QH, UK.


  1. Department of Genetics, University of Liverpool, Liverpool L69 3BX, UK

    • N. Ramsey
    •  & D. A. Ritchie


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