The PhoR/PhoP two-component system of Streptomyces lividans was previously shown to allow the growth of the bacteria at low P i concentrations and to negatively control antibiotic production. The present study focuses on the transcriptional analysis of phoR and phoP, along with the phoU and mtpA genes that are transcribed divergently from the phoRP operon in S. lividans. The effect of phoR, phoP, phoU, and ppk mutations on transcription of these genes was examined under phosphate-replete and phosphate-limited conditions. We demonstrated that phoR and phoP were cotranscribed as a leaderless bicistronic transcript cleaved at discrete sites toward the 3 end of phoR. In addition, phoP could also be transcribed alone from a promoter located at the 3 end of phoR. The phoU and mtpA genes, predicted to encode metal binding proteins, were shown to be transcribed as monocistronic transcripts. The expression of phoR-phoP, phoP, and phoU was found to be induced under conditions of P i limitation in S. lividans TK24. This induction, requiring both PhoR and PhoP, was significantly weaker in the phoU mutant but much stronger in the ppk mutant than in the parental strain. The expression of mtpA was also shown to be up-regulated when P i was limiting but independently of PhoR/PhoP. The induction of mtpA expression was much stronger in the phoU mutant strain than in the other strains. This study revealed interesting regulatory interactions between the different genes and allowed us to propose putative roles for PhoU and MtpA in the adaptation to phosphate scarcity.Phosphate (P i ) is a crucial component of all living organisms, as most of the essential cellular constituents, including ATP, contain P i . Considering the vital importance of P i and its scarcity in the natural world, bacteria have evolved manifold strategies to cope with limited availability of P i . The bacteria first sense P i limitation in the growth medium and then trigger strategies aimed at scavenging and transporting trace amounts of P i that is usually present in the growth medium as metal phosphates (4,35) or at recycling the phosphate present in some phosphate-rich cellular constituents (2, 10). These strategies allow the growth of the bacteria at low P i concentrations. In Escherichia coli or Bacillus subtilis, the two-component systems (PhoB/PhoR and PhoP/PhoR, respectively) were shown to be involved in sensing P i limitation (20,28). An as-yetunknown signal, indicating a P i limitation, triggers autophosphorylation of the sensory kinase on a histidine residue. This phosphoryl group is then transferred to a conserved aspartate residue in the N-terminal receiver domain of a response regulator, increasing the affinity of the latter for its target sites, the pho box, usually present in the promoter regions of genes of the pho regulon (5, 33).Recently, the genes encoding the two-component system of Streptomyces lividans 1326, phoR (encoding a 426-amino-acid [aa]-long sensor kinase) and phoP (encoding a 223-aa-long response regulator), related to analogous sy...