Abstract
Mutants in gene U of phage λ produce polytails. Those polytails have a tail fiber and a basal part like normal tails, but their major tubular part is longer than that of normal tails and shows a wide length distribution.
We established the morphogenetic pathway of the λ tail and found that U gene product (pU) acts throughout the assembly of the major tail protein (pV). Polytails in U^−^ lysate are activated by pU in vitro and form long‐tailed phage which are infectious to a small extent.
If the formation of the basal part of the tail is blocked, pV (the major tail protein) remains unassembled as long as pU is present in the cell. However, we found that part of pV assembles into giant polytubes of several microns in length in lysates of a double mutant U^−^ · H^−^ in which both the basal part and pU are absent.
pV in purified tails can be dissociated into disks (about 10S) or smaller units (about 2.5S) in vitro under extreme conditions. The disks form polytubes efficiently under physiological conditions, but the smaller units do not form polytubes efficiently. The smaller units have in vitro complementation activity with V^−^ lysate. In vitro complementation activities with V^−^, U^−^, and Z^−^ lysates are detected in the dialyzed extracts of SDS gel electrophoresis of purified tails. The molecular weights of the polypeptide chains containing those activities are estimated to be 31,000, 14,000 and 20,000 daltons, respectively.