Study of expression and function of SepL, a regulator of type 3 secretion in enterohaemorrhagic escherichia coli O157
Enterohaemorrhagic Escherichia coli (EHEC) are a recently emerged group of pathogens that can cause fatal infections in the young and elderly. EHEC utilize a virulence factor delivery organelle called a ’Type 3 secretion system’ that results in the formation of characteristic ‘pedestal structures’ on epithelial cells allowing colonization in the human or ruminant gastrointestinal tract. To achieve this, effector proteins have to be injected into host cells. The SepL-SepD complex has been shown to be key for controlling T3-related protein secretion in EHEC. Lack of either protein results in effector hypersecretion and strongly impaired secretion of EspADB translocon proteins. Therefore, the expression and function of SepL was the focus of my PhD research. The expression of SepL was shown to be heterogeneous and co-expressed with EspA filaments in EHEC O157 strains. My work revealed two transcriptional regulators (Ler and SepD) and two putative posttranscriptional regulators (Hfq and CsrA) of SepL expression. Further experiments mapped a key mRNA region required for heterogeneous expression of SepL. This sequence forms a predicted hairpin structure around the Shine-Dalgarno (SD) site of sepL. A model has been formed based on my data in which Hfq and CsrABCD bind to the mRNA potentially competing to control translation. Functionally, the C-terminus of SepL was found to be expendable for 1) SepD binding; 2) SepL membrane localization and 3) translocon export, however it was required for 1) limiting effector secretion via (2) a Tir interaction which might be disassociated by (3) an EscD interaction once host cell signals are sensed. Previously, the concept of two different types of T3 secretion signal were demonstrated in Yersinia spp, I tested this hypothesis in EHEC using both wild type and SepL/SepD deficient EHEC strains. SepL/SepD is required for the N-terminal signal pathway but not a chaperone binding domain signal pathway. A 12aa NleA which only contained an N-terminal signal was shown to bind to SepD and so did the multi-functional T3 chaperone ― CesT. Finally, Far-Western assays demonstrated that SepL only interacted with Tir while SepD could bind other effector proteins indicating that SepL/SepD may act as a targeting hub for effector protein secretion.