Evolutionary and functional characterization of the chimeric enzyme eliminase (ElmA) in Escherichia coli K5

Bacteriophages and bacteria engage in an ancient evolutionary arms race that drives molecular innovation and genetic diversification. Bacteria evolve resistance mechanisms while phages counter with escape mutations, generating diverse defense and counter-defense systems. Within this evolutionary framework, horizontal gene transfer (HGT) enables bacteria to acquire immune mechanisms and repurpose phage-derived elements into host-beneficial functions. Here, we report the characterization of Eliminase (ElmA), a chimeric enzyme in Escherichia coli O10:K5(L):H4 that exemplifies this evolutionary strategy by converting phage weaponry into a bacterial shield. Through integrated phylogenetic, structural, and functional analyses, we demonstrate that ElmA originated from recombination between bacteriophage K5A's tailspike lyase KflA and tail fiber domains-a previously undocumented mechanism generating a host-beneficial capsular regulator from phage lytic machinery. Genomic island analysis positioned elmA within a prophage-derived genetic cassette, while sequence comparisons revealed high similarity between ElmA's N-terminal region and phage tail fiber proteins. Isothermal titration calorimetry demonstrated that the N-terminal domain binds heparosan with K_d of 37.8 μM, accommodating approximately five disaccharide units per protein molecule. Substrate specificity analysis revealed ElmA exhibits strict preference for heparosan, with activity dramatically reduced by N-position modifications. Functional characterization using ElmA-deficient and overexpressing strains revealed a novel regulatory role in capsular polysaccharide trafficking. ElmA facilitates export of low molecular weight heparosan fragments while controlling capsular thickness, functioning as a molecular rheostat modulating polysaccharide flux. These findings illuminate how bacteria co-opt phage-derived enzymes to create sophisticated regulatory systems, transforming viral lytic machinery into host-beneficial functions.