The properties and regulation of the
pstSCAB-encoded Pi uptake system from the alfalfa symbiont Sinorhizobium melilotiare reported. We present evidence that the pstSCABgenes and the regulatory phoUBgenes are transcribed from a single promoter that contains two PhoB binding sites and that transcription requires PhoB. S. melilotistrain 1021 (Rm1021) and its derivatives were found to carry a C deletion frameshift mutation in the pstCgene (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 pstC1021mutation 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 pstC1021allele was shown to generate a partial phoregulon 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 phoCDETmutants was found to be dependent on the pstC1021mutation, as Rm1021 phoCDETmutants formed small white nodules on alfalfa that failed to reduce N2, whereas phoCDETmutant strains with a corrected pstCallele (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 pstSCABgenes nor the phoCDETgenes. Thus, metabolism of alfalfa nodule bacteroids is not Pi limited.