Studies on the control and activity of the Salmonella spi-1 regulator HilD

Salmonella Typhimurium utilizes a type three secretion system (T3SS) encoded on the Salmonella pathogenicity island 1 (SPI1) to invade intestinal epithelial cells and induce inflammatory diarrhea. The level of SPI1 T3SS gene expression is controlled by the transcriptional activator HilA, encoded on SPI1. Expression of hilA is controlled by the transcription factors HilD, HilC and RtsA, which act in a complex feed-forward regulatory loop. The three transcription factors HilD, HilC, and RtsA, belong to the AraC/XylS family and exhibit homology. These regulators also activate the hilD, hilC, and rtsA genes by binding to the same DNA sequences upstream of these promoters. Despite the apparent redundancy in function, HilD has a unique role in SPI1 regulation because the majority of external regulatory inputs act exclusively through HilD. To better understand SPI1 regulation, the nature of interaction between HilD, HilC, and RtsA has been characterized using biochemical and genetic techniques. Our results show that HilD, HilC, and RtsA can form heterodimers as well as homodimers in solution. Comparison with other AraC family members identified a putative α-helix in the N-terminal domain, which acts as the dimerization domain. Alanine substitution in this region results in reduced dimerization of HilD and HilC and also affects their ability to activate hilA expression. The dimer interactions of HilD, HilC, and RtsA add another layer of complexity to the SPI1 regulatory circuit, providing a more comprehensive understanding of SPI1 T3SS regulation and Salmonella pathogenesis. The nucleoid-associated protein H-NS is a xenogenic silencer that has a major effect on SPI1 expression. In this work, we use genetic techniques to show that disruptions of the chromosomal region surrounding hilD have a cis-effect on H-NS-mediated repression of the hilD promoter; this effect occurs asymmetrically over ~4 kb spanning the prgH-hilD intergenic region. Chloramphenicol acetyltransferase cassettes inserted at various positions in this region are also silenced in relation to the proximity to the hilD promoter. We identify a putative H-NS nucleation site, mutation of which results in de-repression of the locus. Furthermore, we genetically show that HilD abrogates H-NS-mediated silencing to activate the hilD promoter. In contrast, H-NS-mediated repression of the hilA promoter, downstream of hilD, is through its control of HilD, which directly activates hilA transcription. Likewise, activation of the prgH promoter, although in a region silenced by H-NS, is strictly dependent on HilA. In summary, we propose a model in which H-NS nucleates within the hilD promoter region to polymerize and exert its repressive effect. Thus, H-NS-mediated repression of SPI1 is primarily through control of hilD expression, with HilD capable of overcoming H-NS to autoactivate.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Microbiology

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Kalafatis, Marinos

Contributors dc:contributor

• Slauch, James M
• Orlean, Peter AB
• Cronan, John E
• Metcalf, William W

• Microbiology

Statement dc:rights

• Copyright 2021 Marinos Kalafatis

Language dc:language

en, eng

Handle dc:identifier

http://hdl.handle.net/2142/113871

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F113871