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Journal of Bacteriology, January 2009, p. 298-309, Vol. 191, No. 1
0021-9193/09/$08.00+0     doi:10.1128/JB.01115-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.

HPrK Regulates Succinate-Mediated Catabolite Repression in the Gram-Negative Symbiont Sinorhizobium meliloti{triangledown}

Catalina Arango Pinedo and Daniel J. Gage*

University of Connecticut, Department of Molecular and Cell Biology, 91 N. Eagleville Rd., U-3125, Storrs, Connecticut 06269-3125

Received 8 August 2008/ Accepted 10 October 2008

The HPrK kinase/phosphatase is a common component of the phosphotransferase system (PTS) of gram-positive bacteria and regulates catabolite repression through phosphorylation/dephosphorylation of its substrate, the PTS protein HPr, at a conserved serine residue. Phosphorylation of HPr by HPrK also affects additional phosphorylation of HPr by the PTS enzyme EI at a conserved histidine residue. Sinorhizobium meliloti can live as symbionts inside legume root nodules or as free-living organisms and is one of the relatively rare gram-negative bacteria known to have a gene encoding HPrK. We have constructed S. meliloti mutants that lack HPrK or that lack key amino acids in HPr that are likely phosphorylated by HPrK and EI. Deletion of hprK in S. meliloti enhanced catabolite repression caused by succinate, as did an S53A substitution in HPr. Introduction of an H22A substitution into HPr alleviated the strong catabolite repression phenotypes of strains carrying {Delta}hprK or hpr(S53A) mutations, demonstrating that HPr-His22-P is needed for strong catabolite repression. Furthermore, strains with a hpr(H22A) allele exhibited relaxed catabolite repression. These results suggest that HPrK phosphorylates HPr at the serine-53 residue, that HPr-Ser53-P inhibits phosphorylation at the histidine-22 residue, and that HPr-His22-P enhances catabolite repression in the presence of succinate. Additional experiments show that {Delta}hprK mutants overproduce exopolysaccharides and form nodules that do not fix nitrogen.

* Corresponding author. Mailing address: University of Connecticut, Department of Molecular and Cell Biology, 91 N. Eagleville Rd., U-3125, Storrs, CT 06269-3125. Phone: (860) 486-3092. Fax: (860) 486-4331. E-mail: daniel.gage{at}uconn.edu

{triangledown} Published ahead of print on 17 October 2008.

Journal of Bacteriology, January 2009, p. 298-309, Vol. 191, No. 1
0021-9193/09/$08.00+0     doi:10.1128/JB.01115-08
Copyright © 2009, American Society for Microbiology. All Rights Reserved.





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