P-279 - Evolutionary dynamics of tailocin tape measure protein length in Pseudomonas viridiflava

The Arabidopsis thaliana phyllosphere hosts diverse bacteria, including the opportunistic pathogen Pseudomonas viridiflava. This bacterium uses tailocins—phage tail-like bacteriocins—to kill closely related strains. Understanding how tailocin-mediated competition shapes microbial communities requires insight into the evolutionary constraints on individual tailocin genes. Within the tailocin gene cluster, the tape measure protein (TMP) determines tailocin size, but its evolutionary dynamics and role in antibacterial activity remain unclear. Among 1350 P. viridiflava isolates from A. thaliana, we identified two dominant TMP length variants, with the shorter variant featuring two deletions in tryptophan-rich alpha-helical repeat regions. The core phylogeny of P. viridiflava showed that these TMP variants have independently emerged multiple times. Moreover, analysis of A. thaliana herbarium metagenomes shows these variants have persisted for at least 200 years. Tailocin sensitivity assays revealed that both TMP variants kill other P. viridiflava strains, with no effect of TMP length on killing specificity. Additionally, P. viridiflava strains (N=5) from Marchantia polymorpha belong to the same lineage as those infecting A. thaliana and exclusively carry the short TMP variant. Our findings suggest that shared structural constraints shape TMP length variants and tailocin function, laying the basis to explore TMP length effects on tailocin killing strength in culture and in planta.