Stephen K Farrand

Stephen K Farrand

Office: (217) 333-1524
Lab: (217) 244-3229

Mail to: Department of Microbiology
601 S Goodwin
Urbana, IL 61801

Professor of Microbiology

Research Topics

Genetics, Host-Pathogen Interactions, Microbial Physiology, Molecular Evolution, Protein-Nucleic Acid Interactions, Regulation of Gene Expression, Sensory Processing, Signal Transduction


A.B. (Biology), Whitman College, 1967
Ph.D. (Microbiology) University of Rochester, 1973
Postdoctoral (Microbiology), University of Washington, 1972-1975

Teaching Interests

Molecular biology of the Agrobacterium-plant interaction

My laboratory is interested in the biology and the molecular biology of the plant pathogen, Agrobacterium tumefaciens. This organism causes cancerous tumors, called crown galls, on susceptible plants. Tumors result from the transfer of a small piece of DNA, called the T-DNA, from the bacterium to the plant cell. The T-DNA becomes integrated into plant cell nuclear DNA and expression of genes on this segment causes the normal plant cell to differentiate into a tumor cell. Expression of additional T-DNA genes causes the tumor cells to produce and secrete novel small carbon compounds called opines. In turn, Agrobacterium cells can utilize opines as sole carbon and energy sources. In the bacterium, the T-DNA and the genes for opine catabolism reside on a large, extrachromosomal virulence element called the Ti plasmid. This plasmid itself is transmissible from the bacterium to recipient bacteria by a mating process called conjugation. Plant signaling and Ti plasmid conjugation. T-DNA transfer is initiated when the bacterium senses a proper chemical signal produced by a wounded plant. Similarly, Ti plasmid conjugal transfer requires a specific plant signal, in this case an opine secreted by the crown gall tumor. These signalling events are crucial to the interaction between Agrobacterium and its host plant, and also between this bacterium and other related bacteria in the plant rhizosphere (Fig. 1).

We are investigating the nature and the mechanism of the signaling events that lead to Ti plasmid conjugal transfer. Using approaches ranging from physiology through genetics to molecular biology, we have dissected the communication system operative in signalling bacterial donor cells to initiate Ti plasmid transfer. We have identified, mapped and sequenced the loci of the Ti plasmid required for conjugal transfer and opine utilization. We have identified the mechanism by which Agrobacterium recognizes and processes the opine signal, and we have cloned and sequenced the genes involved in this regulatory process. Several years ago we discovered that bacterium-to-bacterium signaling between members of the donor population is required for induction of conjugal transfer. We have identified the components of this quorum-sensing system and determined the hierarchical link between opine regulation and the second messenger system. We also identified another regulatory component, an antiactivator called TraM, that is essential for the quorum-dependent nature of the regulatory system.

We currently are investigating the mechanism by which the second messenger, called AAI, activates TraR, the quorum-sensing transcription factor. We are mapping to the crystal structure of this protein amino acid residues involved in signal binding, dimerization, and transcriptional activation (Fig. 2). We also are investigating the mechanism by which TraM prevents gene activation by TraR. Our recent work indicates that TraM specifically binds TraR and acts as a chaperone to target TraR for proteolysis by Lon protease. Our goals are to determine at the molecular level the structure of TraR, the influence of AAI on this structure that allows the activator to dimerize, and the molecular interactions that occur when TraM binds TraR. Finally, we are interested in the physiology of the quorum-sensing system. In particular, we are investigating the role of eflux pumps in the active secretion of AAI, and the mechanism by which AAI returns to the cell to interact with TraR.

Representative Publications

Qin, Y., Su, S., and Farrand, S.K. 2007. Molecular basis of transcriptional antiactivation: TraM disrupts the TraR-DNA complex through stepwise interactions. J. Biol. Chem., 282:19,979–91. [Abstract]

Kim, J.-G., Park, B.-K., Kim, S.-U., Choi, D., Nahm, B.H., Moon, J.S., Reader, J.S., Farrand, S.K., and Hwang, I. 2006. Bases of biocontrol: sequence predicts synthesis, export and target site of agrocin 84, the trojan horse antibiotic associated with control of crown gall tumorigenesis caused by Agrobacterium tumefaciens. Proc. Natl. Acad. Sci. (USA), 103:8846–51. [Abstract]

Reader, J.S., Ordoukhanian, P.T., Kim, J.-G., de Crécy-Lagard, V., Hwang, I., Farrand, S., and Schimmel, P. 2005. Major biocontrol of plant tumors targets tRNA synthetase. Science, 309:1533. [Abstract]

Qin, Y., Luo, Z.-Q., and Farrand, S.K. 2004. Domains formed within the N-terminal region of the quorum-sensing activator TraR are required for transcriptional activation and direct interaction with RpoA from Agrobacterium. J. Biol. Chem., 279:40844–51. [Abstract]

Qin, Y., Smyth, A.J., Su, S., and Farrand, S.K. 2004. Dimerization properties of TraM, the antiactivator that modulates TraR-mediated quorum-dependent expression of Ti plasmid tra genes. Mol. Microbiol., 53:1471–85. [Abstract]

Luo, Z.-Q., Su, S., and Farrand, S.K. 2003. In situ activation of the quorum-sensing transcription factor TraR by its acyl-homoserine lactone ligand: kinetics and consequences. J. Bacteriol., 185:5665–72. [Abstract]

Luo, Z.-Q., Smyth, A. Gao, P., Qin, Y., and Farrand, S.K. 2003. Mutational analysis of TraR: correlating function with molecular structure of a quorum-sensing transcriptional activator. J. Biol. Chem., 278:13,173–13,182. [Abstract]

Qin, Y., Luo, Z.-Q., Smyth, A.J., Gao, P., Beck von Bodman, S., and Farrand, S.K. 2000. Quorum-sensing signal binding results in dimerization of TraR and its release from membranes into the cytoplasm. EMBO J., 19:5212–21. [Abstract]

Complete Publications List