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Journal of Bacteriology, April 2000, p. 2253-2261, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Domain Structure, Oligomeric State, and Mutational Analysis of PpsR, the Rhodobacter sphaeroides Repressor of Photosystem Gene Expression

Mark Gomelsky,^1, Irene M. Horne,² Hye-Joo Lee,^1, John M. Pemberton,² Alastair G. McEwan,² and Samuel Kaplan^1,*

Department of Microbiology and Molecular Genetics, University of Texas Medical School, Houston, Texas 77030,¹ and Department of Microbiology and Parasitology, University of Queensland, Brisbane, Queensland 4072, Australia²

Received 18 October 1999/Accepted 18 January 2000

The transcription factor PpsR from the facultative photoheterotroph Rhodobacter sphaeroides is involved in repression of photosystem gene expression under aerobic growth conditions. We have isolated a number of spontaneous mutations as well as constructed directed mutations and deletions in ppsR. Repressor activities and the oligomeric state of the wild-type and mutant proteins were assayed. Our results suggest that the wild-type PpsR exists in cell extracts as a tetramer. Analysis of the PpsR mutants confirmed that the carboxy-terminal region of PpsR (residues 400 to 464) is involved in DNA binding. The central region of the protein (residues 150 to 400) was found to contain two PAS domains (residues 161 to 259 and 279 to 367). PAS domains are ubiquitous protein modules involved in sensory transduction as well as in protein-protein interactions. All spontaneously isolated mutations, which significantly impaired repressor activity and which mapped outside the DNA binding region, were positioned in the PAS domains. None of these, however, affected the overall oligomeric state. This implies that the conformation of the PAS domains within the tetramer is critical for repressor activity. Upstream of the first PAS domain resides a putative glutamine-rich hinge (residues 127 to 136) that connects the first PAS domain to the amino-terminal region (residues 1 to 135). The role of the amino terminus of PpsR is not obvious; however, extended deletions within this region abolish repressor activity, thus suggesting that the amino terminus is essential for structural integrity of the protein. We present a model of the domain architecture of the PpsR protein according to which PpsR is comprised of three regions: the carboxy terminus responsible for DNA binding, the central region primarily involved in protein oligomerization and possibly signal sensing, and the amino terminus of unknown function. This model may prove useful for determining the mode of PpsR action.

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^* Corresponding author. Mailing address: Department of Microbiology and Molecular Genetics, University of Texas Medical School, 6431 Fannin St., Houston, TX 77030. Phone (713) 500-5502. Fax: (713) 500-5499. E-mail: skaplan{at}utmmg.med.uth.tmc.edu.

Present address: Department of Molecular Biology, University of Wyoming, Laramie, WY 82071.

Permanent address: Department of Biology, College of Natural Sciences, Dong-A University, Pusan, South Korea.

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Journal of Bacteriology, April 2000, p. 2253-2261, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

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