HrcT Is a Key Component of the Type III Secretion System in Xanthomonas spp. and Also Regulates the Expression of the Key hrp Transcriptional Activator HrpX

The 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 ofhrp genes inXanthomonas is regulated by two key regulatory proteins, HrpG and HrpX. However, the interactions betweenhrp gene products in directing T3SE secretion are largely unknown. Here we demonstrated that HrcT ofX. oryzae pv. oryzicola functions as a T3SS component and positively regulates the expression ofhrpX . Transcription ofhrcT occurs via two distinct promoters; one (T1) is with thehrpB operon and the second (T3) withinhrpB7 Via either promoter T1 or T3, the defect in Hrp phenotype byhrcT deletion was corrected in the presence ofhrcT only fromXanthomonas species but not from other phytopathogenic bacteria. An N-terminally truncated HrcT was able to bind thehrpX promoter 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.