This allows activation of pigA, carA and rap transcription. Rap, which is activated via QS and the phosphate response, can then further activate carA and pigA transcription. This results in upregulation of both Car and Pig production via multiple pathways. Figure learn more 9 The proposed mechanism by P i limitation can upregulate secondary metabolism in Serratia 39006. In response to Pi limitation (or pstS mutation), PhoR activates PhoB by phosphorylation. Active PhoB can then activate click here transcription of smaI, pigA and rap (indicated using

solid arrows). Upregulation of smaI results in activation of the QS regulated genes (pigA, carA and rap), via AHL mediated SmaR derepression (indicated using dashed arrows). Rap then further activates carA and pigA expression (indicated using solid arrows). This results in upregulation of Pig and Car production. Multiple studies have linked Pi limitation to enhanced secondary metabolite production [17]. However,

the complex molecular mechanisms underlying phosphate-mediated regulation have proven difficult to elucidate. Extensive studies in Streptomyces species have shown that PhoPR (PhoBR) activates secondary metabolism in response to Pi limitation, including biosynthesis of undecylprodigiosin, a tripyrrole closely related to Pig [40, 41]. However, in Streptomyces, inactivation of PhoP or deletion of phoPR also activates secondary metabolism [41]. In contrast, deletion of phoB and/or phoR in Serratia 39006 had no impact on secondary metabolism, demonstrating clear differences between the regulatory Fosbretabulin cell line mechanisms employed by these distantly related bacteria. Although

the requirement for increased secondary metabolism under conditions of phosphate limitation is unclear, it has been proposed that enhanced secondary metabolism allows the production of compounds which may, for example, directly antagonise other microorganisms or act as signalling molecules, thereby providing producing organisms with a competitive advantage under nutrient deprived conditions [40, 42, 43]. Conclusion In conclusion, we have established that via the global transcriptional regulators PhoB, SmaR Protein kinase N1 and Rap, multiple inter-linked pathways are acting to upregulate secondary metabolism in Serratia 39006 under conditions of Pi limitation, highlighting the importance of Pig and Car production under these conditions. Methods Bacterial strains, plasmids, phage and culture conditions Bacterial strains and plasmids are listed in Additional File 1[44–49]. Serratia sp. ATCC 39006 derivative strains were grown at 30°C and E. coli strains were grown at 37°C in Luria broth (LB; 5 g l-1 yeast extract, 10 g l-1 bacto tryptone and 5 g l-1 NaCl), minimal media (0.1% w/v (NH4)2SO4, 0.41 mM MgSO4, 0.2% w/v glucose, 40 mM K2HPO4, 14.7 mM KH2PO4, pH 6.9–7.1) or in phosphate limiting (PL) media (0.1% w/v (NH4)2SO4, 0.41 mM MgSO4, 0.2% w/v glucose, 0.1 M HEPES, pH 6.9–7.