FNR is a regulator of Salmonella pathogenicity Island 2 in Salmonella Typhimurium

Abstract

During infection, S. Typhimurium employs Salmonella pathogenicity island (SPI)-encoded type three secretion systems (T3SS) 1 and 2 to invade and survive in host cells. However, expression of SPI-2 is seen at the epithelial border prior to host cell invasion in response to an unknown signal. A zone of relative oxygenation adjacent to the gastrointestinal tract mucosa, caused by diffusion of oxygen from the capillary network has been shown to cause priming of Shigella and Enterotoxigenic E. coli for entry into host cells through the oxygen sensing capabilities of the anaerobic metabolism regulator FNR. However, regulation of SPI-2 by FNR has not been established in S. Typhimurium. Here we show using a combination of RNA-seq and ChIP-seq that SPI-2 is highly expressed in an Δfnr mutant under microaerobic conditions, and that FNR is a direct repressor of SPI-2 genes. Our particular focus on FNR regulation of SPI-2 under microaerobic growth in a glycerol/trimethylamine N-oxide/fumarate minimal medium has revealed that, not only does FNR repress the expression of the SPI-2 encoded type three secretion system apparatus proteins, effectors and chaperones through direct regulation of the SPI-2 response regulator SsrB, but it is also involved in the direct repression of a great number of effectors and virulence relevant sRNAs encoded throughout the chromosome and on the Salmonella virulence plasmid and that growth under aerobic conditions relieved repression. Furthermore, we have shown that the accurate spatiotemporal expression of SPI-2 is integral for maintenance of bacterial fitness, provided additional evidence that oxygen is an important signalling molecule for the control of bacterial motility and demonstrated that FNR is an important regular in both intra- and extracellular environments. Our results demonstrate that S. Typhimurium regulates expression of virulence genes in response to changing oxygen concentrations to prepare for the harsh intracellular environment of host cells using the regulator of anaerobic metabolism FNR. Importantly, analysis of our Δfnr RNA-seq-based transcriptomic data in conjunction with previously published datasets, will provide a more complete picture of mixed regulatory interactions within the cell. We hope that the addition of this data will help in the development of the full understanding of all regulatory interactions and inputs involved in the establishment of S. Typhimurium infection.