Genetic analysis of subunit assembly of the tail fiber of bacteriophage T3.

Bacteriophage T3 virions have six tail fibers composed of the product of gene 17 (gp17). Each tail fiber is a trimer of gp17 polypeptide. To characterize the assembly process of the tail fiber, temperature-sensitive (ts) mutants of gene 17 (ts17) were analyzed by SDS-polyacrylamide gel electrophoresis and by extract complementation. Newly synthesized gp17 polypeptide chains matured to SDS-resistant native trimers with a half time of about 7.5 min at 30 degrees. Although all ts17 mutants had similar plating efficiencies at restrictive temperature (41.5 degrees or 42 degrees), they showed different phenotypes. tsNG75, whose mutation was located in the carboxyl-terminal region of gene 17, was defective in trimer assembly at 41.5 degrees. The ts tail fibers formed at 30 degrees lost the ability to attach to the tail upon treatment at 41.5 degrees. There was a change in temperature sensitivity of tsNG75 tail fibers upon attachment to the tail, suggesting that the tail fiber may change conformation after attachment to the tail. tsNG215 and tsNG169, whose mutation sites were located in the amino-terminal region of gene 17, were not defective in the trimer assembly and attachment to the tail at the restrictive temperature. tsNG215 tail fibers formed at 41.5 degrees appear to be aberrant because they were not active in extract complementation and their attachment to fiberless particles resulted in production of noninfectious phage. Tail fibers produced by cells infected with tsNG169 at the restrictive temperature were active in extract complementation. Phage particles were formed in tsNG169-infected cells at the restrictive temperature. These particles were infectious at the permissive temperature and the mutant was non-infectious only if infection was continued at the restrictive temperature. These phenotypic differences exhibited by different gene 17 mutants may indicate the regions within the gene 17 polypeptide that play a role(s) in the folding and assembly of gp17 and in the biological activity of the mature tail fiber.