The DNA Bridging Protein H-NS and the SsrB Transcription Factor Counteract One Another to Silence and Activate Pathogenicity Island Genes in Salmonella

Abstract

The transcription factor SsrB activates transcription of genes located on Pathogenicity Island 2 (SPI-2) in Salmonella enterica. These gene products are responsible for forming a type-three secretion system that secretes effectors that modify the host macrophage vacuole, enabling Salmonella to replicate and then disseminate to the liver and spleen. Recent studies have reported that pathogenicity island genes are silenced by the nucleoid-like protein H-NS1,2. How transcription factors counter or relieve H-NS silencing is a major focus of study. The sifA gene is located outside of SPI-2 and encodes a product required for maintenance of the Salmonella-containing vacuole, providing an intracellular niche conducive to Salmonella replication and survival. Part of this process involves formation of Salmonella-induced filaments (Sifs). We set out to determine whether SsrB directly activates expression of the sifA gene, and whether H-NS could counter or prevent this interaction. In vitro transcription assays indicate that SsrB directly activates sifA transcription and this stimulation is prevented in the presence of H-NS. SsrB activation requires super-coiled templates; in the presence of linear DNA, no sifA transcripts are observed. Using atomic force microscopy, we show that H-NS forms multiple bridging complexes on super-coiled DNA. Additional experiments are underway to examine the effect of SsrB on these complexes. Supported by NIH GM-58746 and NSF MCB-0613014 to LJK and NUS R144000171712 to Y.J.1. Navarre, W.W. et al., (2006) Selective silencing of foreign DNA with low GC content by the H-NS protein in Salmonella. Science 313:236-238.2. Lucchini, S. et al., (2006) H-NS mediates the silencing of laterally acquired genes in bacteria. PLOS Pathog 2:e81-89.