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 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.
Department of Biology, McMaster University, Hamilton, Ontario, Canada L8S 4K1.
S D
SD
Bardin
R T
RT
Voegele
T M
TM
Finan
D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics* / Q000378:metabolism; D005798:Genes, Bacterial; D017136:Ion Transport; D008969:Molecular Sequence Data; D016366:Open Reading Frames; D009876:Operon; D028044:Phosphate-Binding Proteins; D010710:Phosphates / Q000378:metabolism*; D017354:Point Mutation; D017384:Sequence Deletion; D016962:Sinorhizobium meliloti / Q000235:genetics* / Q000378:metabolism; D013489:Suppression, Genetic
D002352:Carrier Proteins; D028044:Phosphate-Binding Proteins; D010710:Phosphates