A Lrp/AsnC Family Transcriptional Regulator Lrp Is Essential for the Pathogenicity of Dickeya oryzae

Dickeya oryzae causes severe soft rot diseases in a range of important crops. To understand its complicated pathogenic mechanisms, we tried to identify the key virulence regulators through transposon mutagenesis. This led to the identification of a member of the Lrp/AsnC family transcriptional regulators in D. oryzae EC1, designated as Lrp. Phenotype analyses showed that Lrp positively regulated biofilm formation and the production of zeamines, proteases and polygalacturonases, but negatively regulated bacterial swimming motility. Deletion of lrp caused a drastic attenuation in bacterial virulence, indicating that Lrp is a key regulator in the modulation of D. oryzae pathogenicity. We further showed that the transcription of the lrp gene was negatively regulated by the transcriptional regulators SlyA, Fis and OhrR, and the transcriptional expression of tzpA, ohrR and fis was positively modulated by Lrp. Moreover, we demonstrated that Lrp can directly bind to the promoter regions of zmsA, zmsK, prtG, prtX, pehK, pehX, fis, tzpA and ohrR. DNase I footprinting assay determined that Lrp was capable of binding to a specific site (5'-GTGTAATTATGGGCGTGCTCCGGG-3') in the promoter of zmsA. Furthermore, we found that four amino acid residues of Lrp, L20, L23, G111 and T146, are essential to the biological function of Lrp. Overall, this study demonstrated that Lrp is an essential virulence modulator in D. oryzae and suggested that Lrp can be a potent target for controlling the soft rot diseases caused by D. oryzae.