The properties and regulation of the
pstSCAB-encoded P i 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 P i 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 P i -specific, high-affinity ( K m, 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 P i -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 N 2 , whereas phoCDETmutant strains with a corrected pstCallele (RmP110) formed pink nodules on alfalfa that fixed N 2 like the wild type. Alfalfa root nodules formed by the wild-type RCR2011 strain expressed the low-affinity orfA-pit-encoded P i uptake system and neither the pstSCABgenes nor the phoCDETgenes. Thus, metabolism of alfalfa nodule bacteroids is not P i limited.