HrcT Is a Key Component of the Type III Secretion System in Xanthomonas spp. and Also Regulates the Expression of the KeyhrpTranscriptional Activator HrpX
ABSTRACTThe type III secretion system (T3SS), encoded byhrp(hypersensitive response and pathogenicity) genes in Gram-negative phytopathogenic bacteria, delivers repertoires of T3SS effectors (T3SEs) into plant cells to trigger the hypersensitive response (HR) in nonhost or resistant-host plants and promote pathogenicity in susceptible plants. The expression ofhrpgenes inXanthomonasis regulated by two key regulatory proteins, HrpG and HrpX. However, the interactions betweenhrpgene products in directing T3SE secretion are largely unknown. Here we demonstrated that HrcT ofX. oryzaepv. oryzicola functions as a T3SS component and positively regulates the expression ofhrpX. Transcription ofhrcToccurs via two distinct promoters; one (T1) is with thehrpBoperon and the second (T3) withinhrpB7Via either promoter T1 or T3, the defect in Hrp phenotype byhrcTdeletion was corrected in the presence ofhrcTonly fromXanthomonasspecies but not from other phytopathogenic bacteria. An N-terminally truncated HrcT was able to bind thehrpXpromoter and activate the expression ofhrpX, supporting that HrcT is a positive regulator ofhrpX. A revised model showing the regulatory interactions between HrcT, HrpX, and HrpG is proposed.