Heat inactivation of bacteriophage T7 (sedimentation coefficient 453 S) yields a nucleoprotein structure sedimenting at 40 S (in vitro 40 S structure). Lysates of T7-infected Escherichia coli, made with the nonionic detergent Brij 58, also contain a 40 S nucleoprotein structure (in vivo 40 S structure). Both structures consist of a phage capsid bound to a mature length of T7 DNA which is outside the phage head. The two differ in that the capsid of the in vitro 40 S structure was located at a random distance from the nearest end of the DNA while the capsid from the in vivo structure was always located less than 7% of the mature DNA length from the nearest end. The nearest end for the in vivo structure seems to be the genetic right end of the DNA. Kinetic labeling experiments suggest that the in vivo 40 S structure is either an intermediate in DNA packaging or a breakdown product from such an intermediate. The fast side of the in vivo 40 S peak contained structures in which T7 capsids were attached to DNA which was longer than mature T7 DNA. The largest DNA detected was close to twice the length of mature T7 DNA.
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