Summary

We have isolated viable mutants of bacteriophage lambda that have in-frame deletions in gene H, which codes for a minor tail protein. They produce correspondingly smaller but active gene H protein products and assemble shorter-tailed phage parti-cles. The deficiency in tail length for each mutant corresponds to the calculated shortening of the gene H protein caused by the deletion. These results show that the H protein determines tail length and argue strongly for a scheme in which the H protein is a ruler or template that measures length during tail assembly