Regulation of Vibrio Quorum Sensing in Natural and Competitive Environments

Abstract

Bacteria have the remarkable ability to rapidly and accurately detect and adapt to fluctuations in the environment. Often, transcriptional and post-transcriptional regulatory mechanisms function in concert to tune gene expression patterns that enhance survival and conserve resources under varying conditions. Among the changes bacteria monitor and respond to is the cell density and species composition of the vicinal community. Bacteria accomplish this using quorum sensing (QS), a cell-cell communication process that alters global gene expression patterns to foster the synchronous execution of collective behaviors. QS involves the production, release, accumulation, and group-wide detection of signaling molecules called autoinducers (AIs). The marine bacterium V. harveyi produces and responds to three AIs, which act in parallel. At low cell-density, AI concentrations are low, and a phosphorelay cascade leads to the activation of five small regulatory RNAs called Qrr1-5 that post-transcriptionally regulate target genes, leading to a downstream QS regulon of over 600 genes. Because Qrr1-5 largely function redundantly, the advantages of encoding five qrr genes are not well-understood. This work explores the transcriptional and post-transcriptional regulatory mechanisms controlling the QS regulon and how the V. harveyi QS response is altered in the presence of competing bacterial species. First, a new QS regulator called LuxT is discovered to repress the transcription of one Qrr small RNA, Qrr1. As a repressor of qrr1, LuxT indirectly regulates Qrr1 target mRNAs, demonstrating how Qrr1 can control gene expression independently of Qrr2-5. Second, LuxT is also identified as a global regulator that functions in parallel to QS to control over 400 genes. Finally, a co-culture model between V. harveyi and its competitor Vibrio fischeri is established to study QS interactions in competitive multi-species environments.