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Journal of Bacteriology, July 2006, p. 5204-5211, Vol. 188, No. 14
0021-9193/06/$08.00+0     doi:10.1128/JB.00387-06
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Constitutive Activation of Two-Component Response Regulators: Characterization of VirG Activation in Agrobacterium tumefaciens

Rong Gao, Aindrila Mukhopadhyay, Fang Fang, and David G. Lynn^*

Center for Fundamental and Applied Molecular Evolution, Departments of Chemistry and Biology, Emory University, Atlanta, Georgia 30322

Received 17 March 2006/ Accepted 27 April 2006

Response regulators are the ultimate modulators in two-component signal transduction pathways. The N-terminal receiver domains generally accept phosphates from cognate histidine kinases to control output. VirG for example, the response regulator of the VirA/VirG two-component system in Agrobacterium tumefaciens, mediates the expression of virulence genes in response to plant host signals. Response regulators have a highly conserved structure and share a similar conformational activation upon phosphorylation, yet the sequence and structural features that determine or perturb the cooperative activation events are ill defined. Here we use VirG and the unique features of the Agrobacterium system to extend our understanding of the response regulator activation. Two previously isolated constitutive VirG mutants, VirGN54D and VirGI77V/D52E, provide the foundation for our studies. In vivo phosphorylation patterns establish that VirGN54D is able to accumulate phosphates from small-molecule phosphate donors, such as acetyl phosphate, while the VirGI77V/D52E allele carries conformational changes mimicking the active conformation. Further structural alterations on these two alleles begin to reveal the changes necessary for response regulator activation.

Present address: Physical Biosciences Division, Lawrence Berkeley National Laboratory, Berkeley CA 94720.