An analysis of the contracted sheath structure of bacteriophage Mu

Polymers of contracted sheath particles of bacteriophage Mu were imaged by the negative-staining technique and the electron images were analyzed by optical and digital methods. By means of the three-dimensional reconstruction technique of DeRosier and Klug [Nature (Lond.) 217, 130--134 (1968)] an averaged density map of the sheath structure at a resolution of about 2.0 nm was derived. The sheath is known to consist of one type of protein with a molecular weight of about 52000 [Admiraal and Mellema, J. Ultrastr. Res. 56, 48--64 (1976)]. The interpretation of the map has given information about packing and shape of the protein subunits. One way to describe the structure is by a set of annular rings with 6-fold symmetry. The height of these rings is about 1.8 nm and successive rings in the structure change by about 33 degrees in azimuth. The protein subunit which occupies more than one ring in the polymer, is elongated. The long axis of the protein subunit is at an angle of about 20 degrees with the plane normal to the polymer axis. These data are discussed in relation to changes in the sheath molecules upon contraction.