The t protein of bacteriophage T4 shares with other holins the ability to cause the formation of a lethal membrane lesion which allows the phage endolysin to attack the peptidoglycan. Moreover, T, like other holins, acts in a saltatory manner at a precisely programmed time in the vegetative cycle. Unlike other holins, however, T has the unique ability to postpone its lethal function in response to a secondary infection by a T-even phage during the vegetative cycle. A signal transduction system that responds to the secondary infection is thought to be encoded by some of the numerous r genes, defined by mutations that abolish this lysis-inhibition (LIN) response. The primary structure of T differs from two main structural patterns found in more than 30 orthologous groups of holins. Genetic approaches were taken to probe the t sequence for features involved in membrane localization, functional timing and LIN regulation. Gene fusion analysis indicates that T has a single TMD near the N-terminus, with the bulk of the protein residing in the periplasm. Mapping and phenotypic analysis of deletion and point mutations in t indicates that the periplasmic domain of T is the major determinant of the timing mechanism and is involved in the LIN response.