Dissertation/Thesis Abstract

A functional study of the Staphylococcus aureus chaperone ClpC: Substrate identification and modulation of sigb activity via the anti-sigma factor RsbW
by Graham, Justin Westbrook, Ph.D., University of Arkansas for Medical Sciences, 2015, 194; 3703998
Abstract (Summary)

Staphylococcus aureus is a Gram-positive pathogen that causes widespread infections in hospital and community settings. S. aureus’s ability to cause infection, in ill and healthy individuals, is dependent on a variety of virulence factors coupled with the increased prevalence of antibiotic resistance strains. Our laboratory has previously shown that ClpC regulates capsule gene expression. ClpC is an ATP-dependent Hsp100/Clp chaperone involved in protein quality control in low-GC Gram-positive bacteria. Since ClpC is a chaperone, it is unlikely that it regulates capsule expression by DNA binding. Instead, we hypothesize that ClpC indirectly regulates capsule gene expression through one or more intermediate transcriptional regulators. The goal is to identify the ClpC substrates involved in regulation of capsule production and other virulence factors. A histidine-tagged ClpC variant protein with mutations in both Walker B motifs (ClpCtrap) was created. The associated substrates were co-purified with the ClpCtrap using a cobalt column followed by gel electrophoresis combined with liquid chromatography-tandem mass spectrometry (GeLC-MS/MS). Using this method, we identified 103 potential substrates of ClpC in S. aureus strain Newman. Further studies showed that among the potential substrates, several have regulatory functions including AgrA, RsbW, CodY, CcpA and Rex. The interaction of ClpC and RsbW was further verified using a bacterial two-hybrid system. RsbW is an anti-sigma factor which functions to sequester the alternative sigma factor, SigB. Because SigB is an alternative sigma factor that affects many genes including those in the capsule operon, we chose to study the interaction of ClpC and RsbW to further understand the mechanism of ClpC-mediated gene regulation. We found that asp23 transcription is repressed by ClpC by quanatative RT-PCR and gene reporter analysis suggesting that the ClpC interaction with RsbW inhibits the ability of SigB to activate transcription (activity). These results allow us to postulate two hypotheses. First, ClpC could protect RsbW from degradation. The increased RsbW amount would increase sequestration of SigB by RsbW leading to reduced SigB activity. Alternatively, ClpC could facilitate RsbW to interact with SigB thereby reducing SigB activity. Preliminary results testing these hypotheses, however, are inconclusive and further studies are currently underway to resolve the issue.

Indexing (document details)
Advisor: Lee, Chia Y.
Commitee: Blevins, Jon, Smeltzer, Mark, Voth, Daniel, Wight, Patricia
School: University of Arkansas for Medical Sciences
Department: Microbiology and Immunology
School Location: United States -- Arkansas
Source: DAI-B 76/09(E), Dissertation Abstracts International
Subjects: Microbiology
Keywords: ClpC chaperone, Gram-positive pathogens, Pathogenesis
Publication Number: 3703998
ISBN: 978-1-321-76139-9
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