Phosphate-Dependent Gene Expression in Free-Living and Symbiotic Sinorhizobium meliloti

Growth of Sinorhizobium meliloti under Pi-deficient conditions induces many genes involved in both P acquisition and in a general response to P-limitation. PhoB regulates genes induced by P limitation by binding to Pho-regulon promoters with a Pho-Box. PhoR is the histidine-protein kinase that activates PhoB by phosphorylation. Pi-PhoB interacts with the 70 subunit of RNA polymerase. Using a whole genome in silico analysis of known Pho-box sequences, we identified 34 genes as putative members of the Pho regulon. Thirty-one genes were induced and three repressed by P-starvation in a PhoB-dependent manner (Yuan et al., 2006a; Krol and Becker, 2004), including genes with no obvious relation to P-metabolism, e.g., for iron transporters and katA, which encodes catalase and has two independent promoters regulated by OxyR and PhoB (Yuan et al., 2005). Pho regulon members in S.meliloti include the pstSCAB and phoCDET operons, encoding ABC-type high-affinity Pi-transporters (Bardin et al., 1998; Voegele et al., 1997; Yuan et al., 2006b), and the orfA-pit operon, which encodes a low-affinity Pi-transport system negatively regulated by PhoB (Bardin et al., 1998). Expression of these S. meliloti Pi transporters in root nodules, using promoter-gusA fusion plasmids in wild type and phoB S. meliloti, showed that the orfA-pit system was highly expressed in nodules, whereas very little phoCDET or pstSCAB expression was detected in either strain. There was no zone-specific expression of orfA-pit::gusA, and neither phoC nor pstS expression was detected in any nodule zone. These and other (Yuan et al., 2005, 2006b) data suggest that bacteroid metabolism in alfalfa nodules is not Pi limited.