Assembly of bacteriophage T4 tail fibers: II. Isolation and characterization of tail fiber precursors

Abstract Complete T4 tail fibers and four precursor structures (half fibers) were purified from lysates of mutant-infected Escherichia coli cells, using serum blocking assays for tail fiber antigens to monitor the isolation procedures. Each of the final preparations showed a single major electrophoretic component on polyacrylamide gels. The purified structures, designated whole fibers and A, BC′, BC, and C half fibers on the basis of their antigenic determinants, were further characterized by electron microscopy and assays for ability to produce viable phage in vitro when combined with appropriate extracts of mutant-infected cells. Purified whole fibers are similar in dimensions and appearance to the tail fibers seen on complete phage, and can be attached quantitatively to tail fiberless particles in vitro to produce active virus. The whole fiber is composed of one A and one BC′ half fiber, each about 690 A in length and 45 A in width, joined end to end at an angle of about 160 °. The isolated A half fiber carries a knob at one end, which is also visible at one end of the whole fiber. Since the A half is proximal to the tail in the completed phage, the knob presumably represents the point of tail fiber attachment to the baseplate. The A half fiber may not be a normal intermediate in whole fiber assembly, since it is not visible in most crude extracts containing A antigen, and retains little in vitro activity through the purification procedure. The BC′ half fiber, by constrast, appears to be the normal precursor of the distal half of the whole fiber; it is visible in crude extracts and retains its in vitro activity in the purified state. BC, the probable precursor of BC′, is inactive in vitro when purified but indistinguishable from BC′ in dimensions and serological properties. C, the probable precursor of BC, is also inactive in vitro when purified. Besides lacking the B antigen it is shorter than BC, being only 560 A in length.