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Journal of Bacteriology, February 2006, p. 1089-1102, Vol. 188, No. 3
0021-9193/06/$08.00+0     doi:10.1128/JB.188.3.1089-1102.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.

Regulation and Properties of PstSCAB, a High-Affinity, High-Velocity Phosphate Transport System of Sinorhizobium meliloti

Ze-Chun Yuan, Rahat Zaheer, and Turlough M. Finan*

Center for Environmental Genomics, Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4R8

Received 16 June 2005/ Accepted 27 October 2005

The properties and regulation of the pstSCAB-encoded Pi uptake system from the alfalfa symbiont Sinorhizobium meliloti are reported. We present evidence that the pstSCAB genes and the regulatory phoUB genes are transcribed from a single promoter that contains two PhoB binding sites and that transcription requires PhoB. S. meliloti strain 1021 (Rm1021) and its derivatives were found to carry a C deletion frameshift mutation in the pstC gene (designated pstC1021) that severely impairs activity of the PstSCAB Pi transport system. This mutation is absent in RCR2011, the parent of Rm1021. Correction of the pstC1021 mutation in Rm1021 by site-directed mutagenesis revealed that PstSCAB is a Pi-specific, high-affinity (Km, 0.2 µM), high-velocity (Vmax, 70 nmol/min/mg protein) transport system. The pstC1021 allele was shown to generate a partial pho regulon constitutive phenotype, in which transcription is activated by PhoB even under Pi-excess conditions that render PhoB inactive in a wild-type background. The previously reported symbiotic Fix– phenotype of phoCDET mutants was found to be dependent on the pstC1021 mutation, as Rm1021 phoCDET mutants formed small white nodules on alfalfa that failed to reduce N2, whereas phoCDET mutant strains with a corrected pstC allele (RmP110) formed pink nodules on alfalfa that fixed N2 like the wild type. Alfalfa root nodules formed by the wild-type RCR2011 strain expressed the low-affinity orfA-pit-encoded Pi uptake system and neither the pstSCAB genes nor the phoCDET genes. Thus, metabolism of alfalfa nodule bacteroids is not Pi limited.

* Corresponding author. Mailing address: Center for Environmental Genomics, Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1. Phone: (905) 525-9140, ext. 22932. Fax: (905) 522-6066. E-mail: finan{at}mcmaster.ca.

Journal of Bacteriology, February 2006, p. 1089-1102, Vol. 188, No. 3
0021-9193/06/$08.00+0     doi:10.1128/JB.188.3.1089-1102.2006
Copyright © 2006, American Society for Microbiology. All Rights Reserved.



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