PHOSPHATE ASSIMILATION IN RHIZOBIUM (SINORHIZOBIUM) MELILOTI - IDENTIFICATION OF A PIT-LIKE GENE

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 sec ond-site mutations can suppress the Fix(-) phenotype of phoCDET mutant s to Fix(+). Here we show that one of these suppressor loci (sfx1) con tains two genes, orfA and pit, which appear to form an operon transcri bed in the order orfA-pit, The Pit protein is homologous to various ph osphate transporters, and we present evidence that three suppressor mu tations arose from a single thymidine deletion in a hepta-thymidine se quence centered 54 nucleotides upstream of the orfA transcription star t site. This mutation increased the level of orfA-pit transcription. T hese data, together with previous biochemical evidence, show that the orfA-pit genes encode a P-i transport system that is expressed in wild -type cells grown with excess P-i but repressed in cells under conditi ons of P-i limitation. In phoCDET mutant cells, orfA-pit expression is repressed, but this repression is alleviated by the second-site suppr essor mutations. Suppression increases orfA-pit expression compensatin g for the deficiencies in phosphate assimilation and symbiosis of the phoCDET mutants.