Casjens, S., E. Wyckoff, M. Hayden, L. Sampson, K.
Eppler, S. Randall, E. T. Moreno, and P. Serwer. 1992. Bacteriophage P22 portal
protein is part of the gauge that regulates packing density of intravirion DNA.
J. Mol. Biol. 224:1055-1074.
The complex double-stranded DNA bacteriophages assemble DNA-free protein
shells (procapsids) that subsequently package DNA. In the case of several
double-stranded DNA bacteriophages, including P22, packaging is associated with
cutting of DNA from the concatemeric molecule that results from replication. The
mature intravirion P22 DNA has both non-unique (circularly permuted) ends and a
length that is determined by the procapsid. In all known cases, procapsids
consist of an outer coat protein, an interior scaffolding protein that assists
in the assembly of the coat protein shell, and a ring of 12 identical portal
protein subunits through which the DNA is presumed to enter the procapsid. To
investigate the role of the portal protein in cutting permuted DNA from
concatemers, we have characterized P22 portal protein mutants. The effects of
several single amino acid changes in the P22 portal protein on the length of the
DNA packaged, the density to which DNA is condensed within the virion, and the
outer radius of the capsid have been determined. The results obtained with one
mutant (NT5/1a) indicate no change (+/- 0.5%) in the radius of the capsid, but
mature DNA that is 4.7% longer and a packing density that is commensurately
higher than those of wild-type P22. Thus, the portal protein is part of the
gauge that regulates the length and packaging density of DNA in bacteriophage
P22. We argue that these findings make models for DNA packaging less likely in
which the packing density is a property solely of the coat protein shell or of
the DNA itself.
