CAREER: Effects of Reductive Evolution on Amino Acid Stress Responses in Bacteria

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Most bacteria cope with starvation by launching a global change in gene expression known as the stringent response. The stringent response causes growth-related genes to be turned off and genes that are needed for survival under nutrient-limiting conditions to be activated. This response is absent in obligate intracellular human pathogens such as Chlamydia, which only grow inside a host cell. The aim of this project to understand how these bacterial pathogens cope with starvation, since they lack the stringent response and yet they are often able to survive amino acid starvation, which is induced by the human host to combat the infection. This project will address fundamental questions about the evolution of pathogens and the unique manner in which they respond to starvation. More practically, it will result in the identification of potential drug and diagnostic targets to treat and identify diseases caused by pathogens such as Chlamydia. In an effort to broaden participation, several strategies will be used to increase participation of American Indians in college-level research. Some of these students will present their research to their alma mater high schools to help encourage the students to pursue college degrees in STEM. In addition, various science-themed activities will be used to increase interest/awareness/participation of American Indian high school and college students in STEM. This study aims to understand the adaptations and their consequences in evolving to obligate intracellular growth. The obligate intracellular bacterium Chlamydia has minimized its genome by eliminating the stringent response and amino acid biosynthesis pathways. The stringent response is activated when an uncharged tRNA binds in the A site of the ribosome, leading to (p)ppGpp synthesis, a global regulator that, in part, regenerates amino acid pools. Chlamydia does not make tryptophan (trp) or (p)ppGpp thus how it survives trp starvation is not well understood. Prior work by the PI has shown that Chlamydia globally increases its transcription in response to trp starvation while globally decreasing its translation. This observation is unusual for a bacterium, and therefore, represents a unique opportunity to advance knowledge. How the amino acid content of a transcript affects gene expression during starvation has not been fully characterized. The hypothesis is that, during trp limitation, chlamydial gene expression is determined by the trp content of the encoded protein. The goals of this project are the following: 1) to determine if the trp content of open reading frames controls transcription during trp starvation; 2) to determine if trp-free proteins are preferentially translated during trp starvation; 3) to compare and contrast the chlamydial response to trp starvation (1&2) with that of other trp auxotrophs, Streptococcus pyogenes (wild-type and stringent mutant) and Coxiella. The major goal of the broader impacts of this project is to increase participation of American Indians in STEM research. In conjunction with the Native Studies Program, the PI also plans to develop a course on science in the context of the American Indian worldview.