Evolution of the chaperone/usher assembly pathway: Fimbrial classification goes Greek

Many Proteobacteria use the chaperonelusher pathway to assemble proteinaceous filaments on the bacterial surface. These filaments can curl intofimbrial or nonfimbrial surface structures (e.g., a capsule or spore coat). This article reviews the phylogeny of operons belonging to the chaperonelusher assembly class to explore the utility of establishing a scheme for subdividing them into clades of phylogenetically related gene clusters. Based on usher amino acid sequence comparisons, our analysis shows that the chaperonelusher assembly class is subdivided into six major phylogenetic clades, which we have termed α-, β-, γ-, κ-, π-, and σ-fimbriae. Members of each clade share related operon structures and encode fimbrial subunits with similar protein domains. The proposed classification system offers a simple and convenient method for assigning newly discovered chaperonelusher systems to one of the six major phylogenetic groups.