| Sinorhizobium meliloti is a soil bacterium (alpha-proteobacteria) that forms N2-fixing root nodules on Medicago sativa (alfalfa). We identified a third inorganic phosphate transporter, the PstSCAB system, in S. meliloti. It is a high affinity and high-velocity phosphate transporter (Km 0.2 muM and Vmax 70 nmoles/min/mg protein). In addition, we determined that the pstC1021 allele was responsible for the previously observed partial constitutive Pho phenotype in S. meliloti strain Rm1021 and its derivatives, and for the nitrogen fixation deficiency phenotype of Rm1021 phoCDET mutants.; To facilitate the study of the PhoB regulated genes in S. meliloti , we undertook complementary approaches including: genome-wide prediction employing a Pho box position weight matrix; experimental validation by gene fusions; comparison with published DNA microarray data; and in silico comparative genomics across 12 closely related proteobacteria. 24 novel Pho regulon members were discovered, including katA, ppk, btaA , and several genes of unknown function: smb20876, smc00801, smc03884, smc01723, smb20843, smc01605, smc03124, and smc04317 (afuA-like). Our data suggest that Pho regulons overlap and interact with several other control circuits in manners hitherto unanticipated. Furthermore, we identified katA as a novel member of the Pho regulon in Sinorhizobium meliloti, Pseudomonas aeruginosa and Agrobacterium tumefaciens. We demonstrated that the katA distal promoter (p1) was induced by H2O2 in an OxyR-dependent way, whereas the katA proximal promoter (p2) was induced by low Pi conditions in a PhoB-dependent manner. We also observed that the katA (p1) promoter is expressed and depends on OxyR in root nodules. |
