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Thin aggregative fimbriae synthesis protein Fimbriae are cell-surface protein polymers, of eg. E coli and Salmonella spp, that mediate interactions important for host and environmental persistence, development of biofilms, motility, colonization and invasion of cells, and conjugation. Four general assembly pathways for different fimbriae have been proposed, one of which is extracellular nucleation-precipitation (ENP), that differs from the others in that fibre-growth occurs extracellularly. Thin aggregative fimbriae (Tafi) are the only fimbriae dependent on the ENP pathway. Tafi were first identified in Salmonella spp and the controlling operon termed agf; however subsequent isolation of the homologous operon in E coli led to its being called csg. Tafi are known as curli because, in the absence of extracellular polysaccharides, their morphology appears curled; however, when expressed with such polysaccharides their morphology appears as a tangled amorphous matrix. The gene agfC is found to be transcribed at low levels, localized to the periplasm in a mature form, and in combination with AgfE is important for AgfA extracellular assembly, which facilitates the synthesis of Tafi. The genes involved in Tafi production are organized into two adjacent divergently transcribed operons, agfBAC and agfDEFG, both of which are required for biosynthesis and assembly. |
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