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Journal of Bacteriology, August 1998, p. 4219-4226, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Phosphate Assimilation in Rhizobium (Sinorhizobium) meliloti: Identification of a pit-Like Gene

Sylvie D. Bardin, Ralf T. Voegele, and Turlough M. Finan*

Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1

Received 10 September 1997/Accepted 1 June 1998

Rhizobium meliloti mutants defective in the phoCDET-encoded phosphate transport system form root nodules on alfalfa plants that fail to fix nitrogen (Fix-). We have previously reported that two classes of second-site mutations can suppress the Fix- phenotype of phoCDET mutants to Fix+. Here we show that one of these suppressor loci (sfx1) contains two genes, orfA and pit, which appear to form an operon transcribed in the order orfA-pit. The Pit protein is homologous to various phosphate transporters, and we present evidence that three suppressor mutations arose from a single thymidine deletion in a hepta-thymidine sequence centered 54 nucleotides upstream of the orfA transcription start site. This mutation increased the level of orfA-pit transcription. These data, together with previous biochemical evidence, show that the orfA-pit genes encode a Pi transport system that is expressed in wild-type cells grown with excess Pi but repressed in cells under conditions of Pi limitation. In phoCDET mutant cells, orfA-pit expression is repressed, but this repression is alleviated by the second-site suppressor mutations. Suppression increases orfA-pit expression compensating for the deficiencies in phosphate assimilation and symbiosis of the phoCDET mutants.

* Corresponding author. Mailing address: Department of Biology, McMaster University, 1280 Main St. West, Hamilton, Ontario, Canada L8S 4K1. Phone: (905) 525-9140, ext. 22932. Fax: (905) 522-6066. E-mail: FINAN{at}MCMASTER.CA.

Journal of Bacteriology, August 1998, p. 4219-4226, Vol. 180, No. 16
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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