The SdiA-Regulated Gene srgE Encodes a Type III Secreted Effector

Abstract

Salmonella enterica serovar Typhimurium is a food-borne pathogen that causes severe gastroenteritis. The ability of Salmonella to cause disease depends on two type III secretion systems (T3SSs) encoded in two distinct Salmonella pathogenicity islands, 1 and 2 (SPI1 and SPI2, respectively). S. Typhimurium encodes a solo LuxR homolog, SdiA, which can detect the acyl-homoserine lactones (AHLs) produced by other bacteria and upregulate the rck operon and the srgE gene. SrgE is predicted to encode a protein of 488 residues with a coiled-coil domain between residues 345 and 382. In silico studies have provided conflicting predictions as to whether SrgE is a T3SS substrate. Therefore, in this work, we tested the hypothesis that SrgE is a T3SS effector by two methods, a ␤-lactamase activity assay and a split green fluorescent protein (GFP) complementation assay. SrgE with ␤-lactamase fused to residue 40, 100, 150, or 300 was indeed expressed and translocated into host cells, but SrgE with ␤-lactamase fused to residue 400 or 488 was not expressed, suggesting interference by the coiled-coil domain. Similarly, SrgE with GFP S11 fused to residue 300, but not to residue 488, was expressed and translocated into host cells. With both systems, translocation into host cells was dependent upon SPI2. A phylogenetic analysis indicated that srgE is found only within Salmonella enterica subspecies. It is found sporadically within both typhoidal and nontyphoidal serovars, although the SrgE protein sequences found within typhoidal serovars tend to cluster separately from those found in nontyphoidal serovars, suggesting functional diversification.

Non-native autoinducer analogs capable of modulating the SdiA quorum sensing receptor in Salmonella enterica serovar Typhimurium

“…Interestingly, these species do not have LuxI-type synthases and do not produce AHLs; thus, SdiA represents an orphan [ 14 ] or “solo” LuxR-type receptor, a class that is rapidly growing in number [ 15 ]. SdiA from the common foodborne pathogen, S. enterica serovar Typhimurium ( S. Typhimurium hereafter), has been a target of research [ 16 – 19 ] and has high sequence identity with SdiA from other genera: for example, S. Typhimurium (GeneBank AAC08299.1) SdiA is 72% identical to Escherichia coli (GeneBank AWF10864.1) SdiA, 67% identical to Klebsiella pneumoniae (GeneBank CDO1572.1) SdiA, 71% identical to Enterobacter clocae (GeneBank AFN80302.1) SdiA, and 84% identical to Citrobacter koseri (GeneBank SQB29462.1) SdiA.…”

Host cell type-dependent translocation and PhoP-mediated positive regulation of the effector SseK1 of Salmonella enterica

“…More than 30 effectors are secreted through Salmonella T3SSs ( Ramos-Morales, 2012 ; Habyarimana et al, 2014 ). Some of them are encoded in SPI1 or SPI2 but many are encoded outside the islands.…”

Direct regulation of the pefI‐srgC operon encoding the Rck invasin by the quorum‐sensing regulator SdiA in Salmonella Typhimurium

“…The second regulation mechanism described is the temperature requirement for the rck locus which is not expressed at temperatures below 30°C (Smith and Ahmer, 2003). Besides the putative 'rck operon', SdiA regulates the transcription of srgE (STM1554), an apparently horizontally acquired gene encoding a type III secreted effector whose function remains unknown (Habyarimana et al, 2014). Interestingly, neither srgE nor the putative 'rck operon' is present in E. coli.…”