The structure and assembly of bacteriophage T4 tailfibers was examined as a model system for studying assemblyof multiprotein structures. The function of six genes (34-38 and 57) is necessary for assembly of tail fibers. Therole of each of these genes in assembly was examined by isolatingand characterizing whole fibers and four precursorswhich accumulate in lysates of mutant phage-infected bacteria.Each isolated structure showed a single major electrophoreticcomponent on polyacrylamide gels indicating near homogeneity.Electron microscopic examination of these isolated structuresrevealed that the whole fiber consists of two halves; one,requiring genes 57 and 34 for its assembly, is a rod 690 x27 Å with a knob on one end and an antigen A; the other,requiring genes 35, 36, 37 and 57 for its assembly, is a rod690 x 26 Å containing antigens B and C (designated BC'). Theantigen B is introduced into this rod by the action of gene36 which increases the length of a half fiber precursor, C,the product of genes 37, 38 and 57, from 560 to 690 Å. Theresulting BC precursor is converted to BC' under the controlof gene 35, a step necessary to allow interaction with theA half fiber, but one making no morphological or serologicalchange in the fiber.
The subunit composition of the isolated fibers andtheir precursors was examined by dissociation at l00°c andgel electrophoresis in the presence of the anionic detergentsodium dodecyl sulfate (SDS). A was found to contain a majorpolypeptide of molecular weight 150,000. C, BC and BC' wereeach found to contain a major polypeptide of molecular weight123,000. Minor components were also present but they werenot reproducible. Because the major polypeptides were solarge they could be resolved on SDS gels of crude lysatesof mutant infected cells. These gels showed that ambermutations in gene 34 eliminated the 150,000 polypeptide andamber mutations in gene 37 eliminated the 123,000 polypeptide.This indicates that these polypeptides are the products ofgenes 34 and 37 (P34 and P37). The other tail fiber genesdid not affect the synthesis of P34 and P37 except thatamber mutations in gene 36 reduced the amount of P37 by40%, suggesting that these genes are co-transcribed. Molecularweight estimates of the fiber precursors show that there aretwo copies of P34 in the A half fiber and two copies of P37in each of the other half fibers.
Mutations in genes 57 and 38 affected the apparentsolubility of P34 and P37 and allow these polypeptides to bedissociated in SDS at 37°C. This is consistent with P57controlling the dimerization of P34 leading to the A halffiber and P38 and P57 controlling the dimerization of P37leading to the C half fiber.
A new apparatus for destaining acrylamide gels electrophoreticallyand a new method of fractionation and scintillationcounting of radioisotope-labeled acrylamide gelsare also described.
