The leucine-responsive regulatory protein SCAB_Lrp modulates thaxtomin biosynthesis, pathogenicity, and morphological development in Streptomyces scabies.

Streptomyces scabies is the best-characterized plant-pathogenic streptomycete, which is a special species among the large genus Streptomyces. The pathogenicity of S. scabies relies on the production of the secondary metabolite thaxtomin A. Little is known about the molecular mechanisms underlying the regulation of thaxtomin biosynthesis in S. scabies beyond the pathway-specific activator TxtR and the cellulose utilization repressor CebR. The leucine-responsive regulatory protein (Lrp) family modulates secondary metabolism in nonpathogenic streptomycetes. However, the regulatory relationship between the Lrp and pathogenic streptomycetes remains unknown. In this study, we demonstrated that SCAB_Lrp (SCAB_77931) from S. scabies significantly affects thaxtomin biosynthesis, pathogenicity, and morphological development. SCAB_Lrp deletion resulted in a dramatic decline in thaxtomin A production and a low-virulence phenotype of S. scabies. An in-depth dissection of the regulatory mechanism of SCAB_Lrp revealed that it positively regulates the transcription of the thaxtomin biosynthetic gene cluster by directly binding to the promoter of the cluster-situated regulator gene txtR. SCAB_Lrp also controls the morphological development of S. scabies by directly activating the transcription of amfC, whiB, and ssgB. SCAB_Lrp directly controls the transcription of its own gene by binding a specific sequence (5'-GGACAGTCGCCGTGCTACG-3'). Moreover, phenylalanine and methionine have been characterized as SCAB_Lrp effectors by strengthening the binding affinity and complex status between SCAB_Lrp and DNA. Our findings characterize a multifunctional regulatory protein, SCAB_Lrp, that controls secondary metabolism, pathogenicity, and sporulation in S. scabies and provide new insights into the complex regulatory network that modulates thaxtomin phytotoxins in pathogenic Streptomyces.