A metrizamide-impermeable capsid in the DNA packaging pathway of bacteriophage T7

Capsids from lysates of bacteriophage T7-infected Escherichia coli have been subjected to buoyant density sedimentation in metrizamide density gradients. Some particles of a T7 capsid previously referred to as capsid II (Serwer, 1976) had a density of 1·275 g/cm³ (metrizamide high-density capsid II or MHD capsid II) and others had a density of 1·086 to 1·100 g/cm³ (metrizamide lowdensity capsid II or MLD capsid II). The densities of MHD and MLD capsids II in cesium chloride density gradients indicate that MLD capsid II has a lower density than MHD capsid II in metrizamide gradients because the preferential solvation, Γ′, of MLD capsid II (2·4 to 1·9 g/g) is higher than the Γ′ of MHD capsid II (0·25 g/g). Data are presented indicating that the higher Γ′ of MLD capsid II occurs because the envelope of MLD capsid II is impermeable and the envelope of MHD capsid II is permeable to metrizamide. Electron microscopy revealed that most MLD capsid II particles had an internal, cylindrical core and that most MHD capsid II particles did not. The results of kinetic labeling experiments suggest that MLD and MHD capsids II are either precursors to the capsid of bacteriophage T7 or degradation products of such precursors. MLD capsid II and MHD capsid II have been characterized by electrophoresis in agarose gels using non-denaturing conditions, sodium dodecyl sulfate/polyacrylamide gel electrophoresis and velocity sedimentation.