Project Summary/Abstract Chlamydia is one of the most important infectious agents from a public health perspective. In 2014 more than 1.4 million cases of chlamydial infections were reported to the CDC making it the most commonly reported infectious disease in the U.S. Chlamydia causes an unusual intracellular infection in which there are two specialized forms of the bacterium within an infected cell. The reticulate body (RB) is an intracellular form that replicates via multiple rounds of binary fission. Then at a late stage in the intracellular infection, each RB asynchronously converts into an elementary body (EB), which is the infectious form that transmits the infection to a new cell. We propose to study how this late developmental change from an RB to an EB is regulated by focusing on a small group of chlamydial genes that are upregulated at late times in the infection. We have evidence that these late genes are negatively regulated to prevent their premature expression. In Aim 1, we will study a transcriptional regulator called EUO to understand how its repression of late genes is relieved at late times to allow these genes to be expressed. We hypothesize that EUO is converted in a redox-dependent manner from a dimer into a monomer that is then degraded by proteolysis. In Aim 2, we will study another regulator called RsbW, which we propose is part of a signaling pathway that controls the transcription of a subset of late genes by σ28 RNA polymerase in response to glucose availability. Aim 3 will use confocal microscopy to examine these mechanisms of late gene regulation in individual RBs and EBs within a single infected cell. We will investigate whether the regulators of late gene expression also control RB- to-EB conversion and why this critical conversion step occurs asynchronously. These studies have the potential to lead to novel therapeutic strategies for treating chlamydial infections by interrupting this critical conversion step and preventing the production of infectious bacteria.