Attachment of tail fibers in bacteriophage T4 assembly: Role of the phage whiskers☆

Abstract The collar and whiskers of bacteriophage T4 extend outward from the top of the tail and play a role in regulating retraction of the tail fibers ( Conley & Wood, 1975 ). The collar and whiskers also are required for efficient tail fiber attachment during phage assembly. The structural gene for the collar/whisker protein is called wac . In vitro , infected-cell extracts that contain tail fibers activate whiskerless ( wac ) tail fiberless particles and ordinary ( wac + ) tail fiberless particles at equal rates if the extracts contain the wac + gene product. However, extracts that contain tail fibers but no wac + gene product activate wac particles about ten times more slowly. In vivo , whiskers are not essential for plaque formation, but a wac mutation causes a delay in the appearance of intracellular phage and a fivefold decrease in the burst size of infectious particles. The effect of the whiskers on tail fiber attachment is due to an interaction between the whisker and the distal half of the tail fiber, similar if not identical to the interaction that controls tail fiber retraction in complete phage. The following observations support this view: a slow rate of in vitro tail fiber attachment similar to that described above is seen with wac + particles when they are pretreated with anti-whisker serum, or when the tail fibers carry a mutational alteration in gp36, a structural protein in the distal half fiber near the central kink. Lack of whiskers does not affect the slow rate of attachment of proximal half fibers to the baseplate of fiberless particles, but lack of whiskers greatly decreases the rate at which particles with attached proximal half fibers are activated by addition of distal half fibers. Since whiskers normally are attached to the phage only after head—tail union ( Coombs & Eiserling, 1977 ; Terzaghi et al. , 1978), these findings explain why tail fibers do not attach efficiently to the baseplates of free tails.