Origin and function of the two major tail proteins of bacteriophage SPP1

Summary The majority of bacteriophages have a long non‐contractile tail ( Siphoviridae ) that serves as a conduit for viral DNA traffic from the phage capsid to the host cell at the beginning of infection. The 160‐nm‐long tail tube of Bacillus subtilis bacteriophage SPP1 is shown to be composed of two major tail proteins (MTPs), gp17.1 and gp17.1*, at a ratio of about 3:1. They share a common amino‐terminus, but the latter species has ∼10 kDa more than gp17.1. A CCC.UAA sequence with overlapping proline codons at the 3′ end of gene 17.1 drives a programmed translational frameshift to another open reading frame. The recoding event generates gp17.1*. Phages carrying exclusively gp17.1 or gp17.1* are viable, but tails are structurally distinct. gp17.1 and the carboxyl‐terminus of gp17.1* have a distinct evolutionary history correlating with different functions: the polypeptide sequence identical in the two proteins is responsible for assembly of the tail tube while the additional module of gp17.1* shields the structure exterior exposed to the environment. The carboxyl‐terminal extension is an elaboration present in some tailed bacteriophages. Different extensions were found to combine in a mosaic fashion with the MTP essential module in a subset of Siphoviridae genomes.