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 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 P-i transport system that is expressed in wild-type cells grown with excess P-i but repressed in cells under conditions of P-i 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.