TRANSCRIPTIONAL REGULATION OF THE IRON-RESPONSIVE SIGMA-FACTOR GENE PBRA

In response to the intracellular iron concentration Pseudomonas fluore scens M114 coordinately regulates the production of pseudobactin M114, its cognate receptor PbuA, and a casein protease. Transcriptional ini tiation of this coordinate iron-stress response requires the sigma fac tor PbrA. PbrA is a member of the ECF (Extracytoplasmic function) subg roup of the sigma(70) family of eubacterial RNA polymerase sigma facto rs. Regulatory studies of the pbr A gene utilising promoter-lacZ trans criptional fusions demonstrate that expression of pbrA dictates the ce llular response to iron. pbrA is transcribed in all phases of iron-lim ited growth but maximally at late-logarithmic to stationary phase. pbr A expression is independent of autoregulatory control but is strictly repressed in iron-rich conditions in a Fur-dependent fashion. Constitu tive expression of pbrA from an inducible tac promoter permits the ind uction of PbrA-dependent transcription and pseudobactin M114 biosynthe sis in high-iron conditions. A PbrA consensus sequence was derived fro m significant DNA sequence homologies observed within the ''-25bp'' an d ''-16bp'' regions conserved among all PbrA-dependent promoters. The predicted PbrA target promoter consensus is homologous for the promote r recognition sites for other environmentally responsive ECF sigma fac tors.