Director of Graduate Studies, Professor of Microbiology
Office: 540B Eckstein Medical Research Building
431 Newton Rd
Iowa City, IA 52242


BS, Biology, University of Wisconsin-Stevens Point
MS, Microbiology, Medical College of Wisconsin
PhD, Microbiology, Medical College of Wisconsin

Post Doctorate, Biochemistry, Dartmouth Medical School

Education/Training Program Affiliations

Department of Microbiology Graduate Program
Interdisciplinary Graduate Program in Translational Biomedicine
Medical Scientist Training Program

Research Summary


My laboratory studies regulation of the Pseudomonas aeruginosa type III secretion system (T3SS). T3SS's are unique to Gram-negative bacteria and function to deliver toxins into eukaryotic host cells. The secretion machinery consists of ~30 proteins which span both the inner and outer membranes of the bacterial cell. Following contact of the bacterium with a eukaryotic cell, the type III machinery functions like a molecular syringe to inject toxins into the host cell. Expression of the P. aeruginosa T3SS is highly regulated and induced by at least two environmental cues; contact of the bacteria with eukaryotic cells and growth in the presence of low Ca2+ concentrations. In the absence of these cues, low amounts of the type III secretion channels are assembled within the cell membranes. The channels, however, are inactive and expression of the T3SS is repressed. Expression of the T3SS genes is coupled to type III secretory activity by a cascade of interacting regulatory proteins (ExsA, ExsD, ExsC, and ExsE). ExsA is an activator of type III gene transcription, ExsD binds ExsA to inhibit transcription, ExsC inhibits ExsD activity, and ExsE inhibits ExsC activity. The entire process is coupled to secretion by virtue of the fact that ExsE is a secreted substrate of the T3SS. Changes in the intracellular concentration of ExsE are thought to govern formation of the ExsC-ExsE, ExsC-ExsD, and ExsD-ExsA complexes. Whereas formation of the ExsC-ExsE complex allows ExsD to bind ExsA and transcription of the T3SS is repressed, formation of the ExsC-ExsD complex sequesters ExsD from ExsA and transcription of the T3SS is induced. The major efforts in my lab involve testing the above model and determining how this regulatory cascade interfaces with a number of additional regulatory mechanisms also controlling T3SS gene expression.


Center, Program and Institute Affiliations

Center for Biocatalysis and Bioprocessing
Center for Gene Therapy of Cystic Fibrosis and other Genetic Diseases

Selected Publications

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Intile P, Balzer G, Wolfgang M, Yahr T.  The RNA helicase DeaD stimulates ExsA translation to promote expression of the Pseudomonas aeruginosa type III secretion system.  J Bacteriol.  2015 August. 197(16):2664-74. 

Ince D, Sutterwala F, Yahr T.  Secretion of Flagellar Proteins by the Pseudomonas aeruginosa Type III Secretion-Injectisome System.  J Bacteriol.  2015 June 15. 197(12):2003-11. 

Marsden A, Schubot F, Yahr T.  Self-association of ExsA is required for occupation of adjacent binding sites on Pseudomonas aeruginosa type III secretion system promoters.  J Bacteriol.  2014 October. 196(20):3546-55. 

Intile P, Diaz M, Urbanowski M, Wolfgang M, Yahr T.  The AlgZR Two-Component System Recalibrates the RsmAYZ Posttranscriptional Regulatory System To Inhibit Expression of the Pseudomonas aeruginosa Type III Secretion System.  J Bacteriol.  2014 January. 196(2):357-66. 

King J, Schesser Bartra S, Plano G, Yahr T.  ExsA and LcrF recognize similar consensus binding sites, but differences in their oligomeric state influence interactions with promoter DNA.  J Bacteriol.  2013 December. 195(24):5639-50. 

Heimer S, Evans D, Stern M, Barbieri J, Yahr T, Fleiszig S.  Pseudomonas aeruginosa Utilizes the Type III Secreted Toxin ExoS to Avoid Acidified Compartments within Epithelial Cells.  PLoS One.  2013 September 18. 8(9):e73111. 

Marden J, Diaz M, Walton W, Gode C, Betts L, Urbanowski M, Redinbo M, Yahr T, Wolfgang M.  An unusual CsrA family member operates in series with RsmA to amplify posttranscriptional responses in Pseudomonas aeruginosa.  Proc Natl Acad Sci U S A.  2013 September 10. 110(37):15055-60. 

Olson M, King J, Yahr T, Horswill A.  Sialic Acid Catabolism in Staphylococcus aureus.  J Bacteriol.  2013 April. 195(8):1779-88. 

King J, Brutinel E, Marsden A, Schubot F, Yahr T.  Orientation of Pseudomonas aeruginosa ExsA monomers bound to promoter DNA and base-specific contacts with the P(exoT) promoter.  J Bacteriol.  2012 May. 194(10):2573-85. 

Brutinel E, King J, Marsden A, Yahr T.  The distal ExsA-binding site in Pseudomonas aeruginosa type III secretion system promoters is the primary determinant for promoter-specific properties.  J Bacteriol.  2012 May. 194(10):2564-72. 

Web: Yahr Lab Website
Web: Carver College of Medicine Interview
Web: Google Scholar Page