Regulation and Properties of PstSCAB, a High-Affinity, High-Velocity Phosphate Transport System ofSinorhizobium melilotiJournal Articles
Overview
Research
Identity
Additional Document Info
View All
Overview
abstract
ABSTRACTThe properties and regulation of thepstSCAB-encoded Piuptake system from the alfalfa symbiontSinorhizobium melilotiare reported. We present evidence that thepstSCABgenes and the regulatoryphoUBgenes 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 thepstCgene (designatedpstC1021) that severely impairs activity of the PstSCAB Pitransport system. This mutation is absent in RCR2011, the parent of Rm1021. Correction of thepstC1021mutation 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. ThepstC1021allele was shown to generate a partialphoregulon 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 ofphoCDETmutants was found to be dependent on thepstC1021mutation, as Rm1021phoCDETmutants formed small white nodules on alfalfa that failed to reduce N2, whereasphoCDETmutant strains with a correctedpstCallele (RmP110) formed pink nodules on alfalfa that fixed N2like the wild type. Alfalfa root nodules formed by the wild-type RCR2011 strain expressed the low-affinityorfA-pit-encoded Piuptake system and neither thepstSCABgenes nor thephoCDETgenes. Thus, metabolism of alfalfa nodule bacteroids is not Pilimited.
