A fragile lattice: replacing bacteriophage λ's head stability gene D with the shp gene of phage 21 generates the Mg2+-dependent virus, λ shp

AbstractPhage λ DNA packaging is accompanied by prohead expansion, due to structural changes in gpE, the major capsid protein. Rearrangement of the gpE lattice creates binding sites for trimers of gpD, the head stabilization protein. λ-Like phage 21's shp gene is homologous to λ's D gene. gpD and gpShp share 49% amino acid identity. To ask whether gpShp could stabilize the λ head shell, we replaced λ's D gene with shp, creating λ shp. Unlike λ or 21, λ shp was strictly dependent on the presence of 10−2 M Mg2+, and λ shp virions were very sensitive to chelating agents. Density gradient studies indicated that the λ gpE lattice was underpopulated with gpShp. gpD's N-terminus has been proposed to contact gpE, and we found that λ D/shp, which produces a chimeric protein with the N-terminus of gpD and the C-terminus of gpShp, was Mg2+-independent and more stable than λ shp