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Author: Guido Sessa
Internal reviewer: David J. Studholme
Expert reviewer: Laurent Noël

Class: XopAE
Family: XopAE
Prototype: HpaF/HpaG (Xanthomonas euvesicatoria pv. euvesicatoria, ex Xanthomonas campestris pv. vesicatoria; strain 85-10)
GenBank ID: AAL78290.1 (197 aa) / AAL78291.1 (432 aa)
RefSeq ID: WP_011050288.1 (547 aa)
Synonyms: XopAE was also referred to as HpaF (Kim et al., 2003; Sugio et al., 2005). In Xcv strain 85-10, the gene is bipartite and the two parts were first coined hpaF and hpaG (Noël et al., 2002). Be careful: hpaG was also used in other Xanthomonas to designate the homolog of hpa1 (X. campestris, X. oryzae) and xopA (Xcv 85-10) (Kim et al., 2003).
3D structure: Most XopAE pocesses a N-terminal leucine-rich repeat (LRR) domain and a C-terminal XL box motif with an E3 ubiquitin ligase fold, based on homology modelling (Popov et al., 2018). Predicted fold of XopAE C terminus is similar to that of XopL (Popov et al., 2018).

The locus

The xopAE locus in the majority of the Xanthomonas strains encodes a single ORF (xopAE). Yet, in X. euvesicatoria strain 85-10, a frame-shift splits it into two ORFs (hpaF and xopAE85-10 ) that are transcribed in an operon as a bicistronic mRNA (Noël et al., 2002; Popov et al., 2018).

Biological function

How discovered?

XopAE was discovered by studying hrpG-regulated genes within the hrp gene cluster of X. euvesicatoria strain 85-10 (Noël et al., 2002).

(Experimental) evidence for being a T3E

XopAE85-10 was fused to the AvrBs2 reporter and shown to translocate into plant cells in an hrpF-dependent manner (Popov et al., 2018).


In X. euvesicatoria strain 85-10, xopAE 85-10 transcription is driven by the promoter of the immediately upstream hpaF gene and regulated by the HrpX and HrpG master transcriptional regulators of the Xanthomonas T3SS (Popov et al., 2016). An imperfect PIP box (TTCGC-N16-TTCGC) is located 82 bp upstream of the predicted translation start codon of hpaF (Noël et al., 2002).


XopAE85-10 is a suppressor of PTI that was shown to inhibit flg22-mediated signaling in Arabidopsis and tomato protoplasts, downstream or in parallel to the activation of the MPK3 and MPK6 MAP kinases. Pseudomonas syringae-mediated delivery of XopAE85-10 in plants inhibits PTI-associated callose deposition at the cell wall and enhances disease symptoms in tomato (Popov et al., 2016).


XopAE85-10-YFP fusion protein localizes to the cytoplasm and nucleus of Nicotiana benthamiana leaf epidermal cells (Popov et al., 2016).

Enzymatic function

XopAE85-10 is an active E3 ubiquitin ligase in vitro (Popov et al., 2018).

Interaction partners



In xanthomonads

Yes (Popov et al., 2018). The xopAE locus is present in sequenced genomes of X. alfalfae, X. euvesicatoria, X. oryzae, X. bromi, X. vasicola and X. fragariae, but not in X. campestris pv. campestris, for instance.

In other plant pathogens/symbionts

Yes. There are homologues in Acidovorax spp. (sharing up to about 50% amino-acid sequence identity) and in Ralstonia spp. (less than 35% sequence identity).


Kim JG, Park BK, Yoo CH, Jeon E, Oh J, Hwang I (2003). Characterization of the Xanthomonas axonopodis pv. glycines Hrp pathogenicity island. J. Bacteriol. 185: 3155‐3166. DOI: jb.185.10.3155-3166.2003

Noël L, Thieme F, Nennstiel D, Bonas U (2002). Two novel type III-secreted proteins of Xanthomonas campestris pv. vesicatoria are encoded within the hrp pathogenicity island. J. Bacteriol. 184: 1340-1348. DOI: 10.1128/JB.184.5.1340-1348.2002

Popov G, Fraiture M, Brunner F, Sessa G (2016). Multiple Xanthomonas euvesicatoria type III effectors inhibit flg22-triggered immunity. Mol. Plant Microbe Interact. 29: 651-660. DOI: 10.1094/MPMI-07-16-0137-R

Popov G, Majhi BB, Sessa G (2018). Effector gene xopAE of Xanthomonas euvesicatoria 85-10 is part of an operon and encodes an E3 ubiquitin ligase. J. Bacteriol. 16: e00104-18. DOI: 10.1128/JB.00104-18

Sugio A, Yang B, White FF (2005). Characterization of the hrpF pathogenicity peninsula of Xanthomonas oryzae pv. oryzae. Mol. Plant Microbe Interact. 18: 546‐554. DOI: 10.1094/MPMI-18-0546

Further reading

Zhao S, Mo WL, Wu F, Tang W, Tang JL, Szurek B, Verdier V, Koebnik R, Feng JX (2013). Identification of non-TAL effectors in Xanthomonas oryzae pv. oryzae Chinese strain 13751 and analysis of their role in the bacterial virulence. World J. Microbiol. Biotechnol. 29: 733-744. DOI: 10.1007/s11274-012-1229-5

bacteria/t3e/xopae.txt · Last modified: 2023/05/24 09:34 by rkoebnik