Botstein, D., Waddell, C.H. and King, J. (1973) J Mol Biol., 80(4), 669-695.

Mechanism of head assembly and DNA encapsulation in Salmonella phage p22. I. Genes, proteins, structures and DNA maturation.


The functions of ten known late genes are required for the intracellular assembly of infectious particles of the temperate Salmonella phage P22. The defective phenotypes of mutants in these genes have been characterized with respect to DNA metabolism and the appearance of phage-related structures in lysates of infected cells. In addition, proteins specified by eight of the ten late genes were identified by sodium dodecyl sulfate/polyacrylamide gel electrophoresis; all but two are found in the mature phage particle. We do not find cleavage of these proteins during morphogenesis.

The mutants fall into two classes with respect to DNA maturation; cells infected with mutants of genes 5, 8, 1, 2 and 3 accumulate DNA as a rapidly sedimentin complex containing strands longer than mature phage length. 5- and 8- lysates contain few phage-related sturcutres. Gene 5 specifies the major head structural protein; gene 8 specifies the major protein found in infected lysates but not in mature particles. 1-, 2- and 3- lysates accumulate a single distinctive class of particle ("proheads"), which are shperical and not full of DNA, but which contain some internal material. Gene 1 protein is in the mature particle, gene 2 protein is not.

Cells infected with mutants of the remaining five genes (10, 26, 16, 20 and 9) accumulate mature length DNA. 10- and 26- lysates accumulate empty phage heads, but examination of freshly lysed cells shows that many were initally full heads. These heads can be converted to viable phage by in vitro complementation in concentrated extracts. 16- and 20- lysates accumulate phage particules that appear normal but are non-infectious, and which cannot be rescued in vitro.

From the mutant phenotypes we conclude that an intact prohead structure is required to mature the virus DNA (i.e. to cut the overlength DNA concatemer to the mature ltnght). Apparently this cutting occurs as part of the encapsulation event.

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