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Troy Hammerstrom

Alumnus

Accepted post-doctoral position at Rice after receiving PhD

Coordinated expression of virulence genes in Bacillus anthracis occurs via a multi-faceted signal transduction pathway that is dependent upon the AtxA protein.  Intricate control of atxA gene transcription and AtxA protein function have become apparent from studies of AtxA-induced synthesis of the anthrax toxin proteins and the poly-D-glutamic acid capsule, two factors with important roles in B. anthracis pathogenesis.  The amino-terminal region of the AtxA protein contains winged-helix (WH) and helix-turn-helix (HTH) motifs, structural features associated with DNA-binding. Using filter binding assays, I determined that AtxA interacted non-specifically at a low nanomolar affinity with a target promoter (Plef) and AtxA-independent promoters.  AtxA also contains motifs associated with phosphoenolpyruvate: sugar phosphotransferase system (PTS) regulation. These PTS-regulated domains, PRD1 and PRD2, are within the central amino acid sequence.  Specific histidines in the PRDs serve as sites of phosphorylation (H199 and H379).  Phosphorylation of H199 increases AtxA activity; whereas, H379 phosphorylation decreases AtxA function. For my dissertation, I hypothesized that AtxA binds target promoters to activate transcription and that DNA-binding activity is regulated via structural changes within the PRDs and a carboxy-terminal EIIB-like motif that are induced by phosphorylation and ligand binding. I determined that AtxA has one large protease-inaccessible domain containing the PRDs and the carboxy-terminal end of the protein. These results suggest that AtxA has a domain that is distinct from the putative DNA-binding region of the protein.

My data indicate that AtxA activity is associated with AtxA multimerization. Oligomeric AtxA was detected when co-affinity purification, non-denaturing gel electrophoresis, and bis(maleimido)hexane (BMH) cross-linking techniques were employed.  I exploited the specificity of BMH for cysteine residues to show that AtxA was cross-linked at C402, implicating the carboxy-terminal EIIB-like region in protein-protein interactions.  In addition, higher amounts of the cross-linked dimeric form of AtxA were observed when cells were cultured in conditions that promote toxin gene expression.  Based on the results, I propose that AtxA multimerization requires the EIIB-like motif and multimerization of AtxA positively impacts function.

Overall, my data support the following model of AtxA function.  AtxA binds to target gene promoters in an oligomeric state. AtxA activity is increased in response to the host-related signal bicarbonate/CO2 because this signal enhances AtxA multimerization.  Future studies will address the interplay between bicarbonate/CO2 signaling and phosphorylation on AtxA function.

Search pubmed for papers by TG Hammerstrom and TM Koehler

Research Info

Functional characterization of ATXA, the Bacillus anthracis virulence regulator