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

Abstract

The type III secretion system (T3SS), encoded by hrp (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 of hrp genes in Xanthomonas is regulated by two key regulatory proteins, HrpG and HrpX. However, the interactions between hrp gene products in directing T3SE secretion are largely unknown. Here we demonstrated that HrcT of X. oryzae pv. oryzicola functions as a T3SS component and positively regulates the expression of hrpX. Transcription of hrcT occurs via two distinct promoters; one (T1) is with the hrpB operon and the second (T3) within hrpB7 Via either promoter T1 or T3, the defect in Hrp phenotype by hrcT deletion was corrected in the presence of hrcT only from Xanthomonas species but not from other phytopathogenic bacteria. An N-terminally truncated HrcT was able to bind the hrpX promoter and activate the expression of hrpX, supporting that HrcT is a positive regulator of hrpX. A revised model showing the regulatory interactions between HrcT, HrpX, and HrpG is proposed.

Comparative proteomic analysis of Xanthomonas citri ssp. citri periplasmic proteins reveals changes in cellular envelope metabolism during in vitro pathogenicity induction

“…In addition, the hrpB operon has been reported to be related to HrcT expression, a key component of the T3SS in Xanthomonas spp. (Liu et al ., ). The corresponding Hrp proteins were not detected in our differential study, possibly because they may have a diverse cell location, as in membranes for example, or are not detectable by our approach, requiring further protein purification (Weber et al ., ).…”

Flp, a Fis‐like protein, contributes to the regulation of type III secretion and virulence processes in the phytopathogen Xanthomonas campestris pv. campestris

“…oryzae (Liu et al , ; Rashid et al , ), a T3SS component HrcT positively regulates the expression of hrpX via binding to its promoter in X. oryzae pv. oryzicola (Liu et al , ), and the zinc uptake regulator Zur and a GntR family transcriptional regulator HpaR1 indirectly control the expression of hrp genes via HrpX and HrpG, respectively, in Xcc (An et al , ; Huang et al , ). Here our experimental evidences suggest a novel regulatory pathway control T3SS in Xcc .…”

HrpB7 from Xanthomonas campestris pv. vesicatoria is an essential component of the type III secretion system and shares features of HrpO/FliJ/YscO family members

“…This is an important control because hrpB7 from Xanthomonas oryzae pv. oryzicola was reported to contain a promoter for the downstream hrcT gene, which encodes an essential component of the export apparatus (Liu et al, ).…”