Prions are infectious, self-propagating protein aggregates that were first described in the context of a group of fatal neurodegenerative diseases known as the transmissible spongiform encephalopathies (TSEs), which afflict humans and other mammals. The protein culprit in the case of the TSEs is an endogenous protein called PrP that has an inherent ability to undergo a dramatic conformational conversion, leading to the formation of distinctive cross-? aggregates (termed amyloid) that are both self-templating and infectious. Prions have also been uncovered in budding yeast and other fungi, where they act as protein-based genetic elements that confer new phenotypes on those cells that harbor them. Like PrP, fungal prion proteins exist in either a native, soluble form or a self-perpetuating, amyloid form (the prion form) that is infectious. Involved in diverse cellular processes, fungal prion proteins can, in their prion forms, enhance cell survival under specific stress conditions. Although prion proteins are widely distributed throughout the fungal kingdom, it is not yet known if they exist in the bacterial domain of life. The foundation for the proposed research is our recent demonstration that E. coli cells can propagate a model yeast prion in a manner that depends on the activity of a conserved cellular chaperone assembly that is also required for prion propagation in yeast. These findings indicate that the basic requirements for protein-based heredity are satisfied in the bacterial domain of life, suggesting that prion-like phenomena may predate the evolutionary split between bacteria and yeast. The proposed research will address this hypothesis through the development of tools and approaches for uncovering prion-like proteins in bacteria, while at the same time investigating the determinants that dictate protein amyloidogenicity. In aim 1, we will systematically investigate the chaperone requirements for prion propagation in E. coli. In aim 2, we will screen bacterially encoded polypeptides for amyloidogenicity, as a means to identify candidate prion-like proteins and also to generate an unbiased experimental data set for evaluating the steric zipper model for protein amyloidogenicity. In aim 3, we will develop and implement a complementary set of approaches to detect prion-like phenomena in bacteria. Together, the proposed experiments will enable a deeper understanding of the cellular requirements for prion formation and propagation, will help elucidate the intrinsic determinants of protein amyloidogenicity and will facilitate the discovery f new protein-based epigenetic sources of phenotypic diversity in the bacterial domain of life.