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Bacterial virulence factors

Plant resistance genes

Molecular Diagnosis and Diversity for Regulated Xanthomonas

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Author: Nay C. Dia
Internal reviewer: Jens Boch
Expert reviewer: Sabine Thieme

Class: AvrBs3
Family: Transcription Activator-Like (TAL) Effectors, TALEs (previously: AvrBs3/PthA)
Prototype: AvrBs3 (Xanthomonas euvesicatoria pv. euvesicatoria, ex Xanthomonas campestris pv. vesicatoria; strain 71-21)
RefSeq ID: P14727.2 (1164 aa)
3D structure: 2KQ5 (Murakami et al., 2010); 3V6P, 3V6T (Deng et al., 2012a); 4GJP, 4GJR (Deng et al., 2012b); 4HPZ (Gao et al., 2012) ; 3UGM (Mak et al., 2012); 4GG4 (Yin et al., 2012); 2YPF (Stella et al., 2013); 4OSH, 4OSI, 4OSJ, 4OSK, 4OSL, 4OSM, 4OSQ, 4OSR, 4OSS, 4OST, 4OSV, 4OSW, 4OSZ, 4OT0, 4OT3, 4OTO (Deng et al., 2014); 6JTQ, 6JVZ, 6JW0, 6JW1, 6JW2, 6JW3, 6JW4, 6JW5 (Liu & Yi, unpublished)

Biological function

How discovered?

The gene avrBs3 was cloned in 1989 and was the first gene described of the TAL effector (TALE) family (Minsavage et al., 1990). Different resistant and susceptible cultivars of peppers were inoculated with Xcv strains 71-21 and 82-8 (Bonas et al., 1989). The pepper cultivar ECW-30R carries the resistance gene Bs3 and inoculation of these Xcv strains provoked a hypersensitive response (HR) (Bonas et al., 1989). This indicated that both Xcv strains contained avrBs3.

(Experimental) evidence for being a T3E

AvrBs3 is secreted and translocated into the plant via the Hrp type III secretion system (Bonas et al., 1991; Van den Ackerveken et al., 1996; Bonas et al., 1999). In contrast to wild-type bacteria, an Xcv mutant carrying a deletion in the conserved hrp gene hrcV did not secrete AvrBs3 indicating that AvrBs3 is transported by the Hrp system (Rossier et al., 1999). The first 10 and 50 amino acids of AvrBs3 are required for secretion and translocation, respectively (Scheibner et al., 2017). In its C-terminal domain, AvrBs3 carries an acidic activation domain which is functional in plant cells (Van den Ackerveken et al., 1996). Two nuclear localization signals in the C-terminal domain of AvrBs3 facilitate transport into the plant cell nucleus (Van den Ackerveken et al., 1996; Szurek et al., 2002). These eukaryotic features support the role of AvrBs3 and members of the TALE family within the eukaryotic host cell.


Unlike most other type III effectors, expression of avrBs3 is not dependend on the hrp regulon and the gene does not contain a PIP box in its promoter region. It is expressed constitutively in cells grown in minimal or complex medium and in planta (Knoop et al., 1991).


AvrBs3, as well as other members of the TALE family, function as specific transcription factors in plant cells. These proteins bind to specific sequences in promoters and induce expression of downstream genes. The DNA-binding specificity is encoded in the order of individual 34-amino acid repeats which each recognize one DNA base. Different TALEs typically contain different repeats and accordingly bind to different DNA sequences and target different host genes. The contributions of individual TALEs to virulence can thus be quite diverse.

Expression analysis using gene promoter fusion and western blot analysis demonstrated that avrBs3 was expressed and resulted in a 122 kDa protein (1164 aa) which was detectable using a specific polyclonal antibody (Bonas et al., 1989). The AvrBs3 effector protein elicits two different types of responses in resistant or susceptible plants. In susceptible pepper plants (Early Cal Wonder; ECW), hypertrophy (i.e. enlargement of mesophyll cells) is triggered by AvrBs3 (Bonas et al., 1989; Bonas et al., 1991; Marois et al., 2002). Agrobacterium strains carrying a vector with avrBs3 induced pustules (hypertrophy) 4-5 dpi in various solanaceous plants including Nicotiana clevelandii, N. benthamiana, N. tabacum, Petunia hybrida, Physalis alkekengi, Solanum americanum and potato (S. tuberosum), whereas Agrobacterium strains carrying an empty vector did not cause any changes in inoculated plants (Marois et al., 2002; Kay et al., 2007). Differential cDNA analysis from susceptible pepper plants infected with Xcv with or without AvrBs3 led to the discovery of upa (upregulated by AvrBs3) genes whose expression is induced by AvrBs3 (Marois et al., 2002; Kay et al., 2007). These UPA genes all share a conserved promoter element, known as the UPA box (Kay et al., 2007). UPA20 acts as a master regulator of cell enlargement causing the hypertrophy symptoms associated with AvrBs3. Silencing of UPA20 decreased cell hypertrophy in infected plants while the expression of UPA20 led to hypertrophy in uninfected plants (Kay et al., 2007).

In resistant pepper plants, the promoter of Bs3 contains a UPA box that is bound by AvrBs3 resulting in the transcription of the gene Bs3. Bs3 encodes a protein that is homologous to flavine-dependent mono-oxygenases (Römer et al., 2007) and its expression causes rapid cell death thus preventing the spread of the pathogen (Bonas et al., 1989; Bonas et al., 1991).

The central region of the avrBs3 gene consists of 17.5 nearly identical 102 bp repeats. Each repeat encodes 34 amino acids (Bonas et al., 1989). Repeat variable di-residues (RVDs) at positions 12 and 13 determine the specificity of each repeat (Boch et al., 2009; Moscou & Bogdanove, 2009). Rearranging individual repeats enables construction of any desired DNA-binding specificity (Boch et al., 2009).


The avrBs3 gene is localized on pXV11, a self-transmissible plasmid, and was initially isolated from Xcv strain 71-21 (Bonas et al., 1989). Using complementation of Xcv strain 85-10 (virulent on pepper ECW-30R), a 5-kb fragment including avrBs3 was discovered (Bonas et al., 1989).

Molecular function

DNA-binding protein. Transcriptional activator.

Interaction partners

Importin alpha (Szurek et al., 2001) interacts with the nuclear localization sequences of AvrBs3. The basal transcription factor IIA, gamma subunit from rice interacts with a region in the C-terminal domain of TALEs (Yuan et al., 2016) and similar interactions might be possible for AvrBs3, too. AvrBs3 and the TALE-family of effectors bind to DNA (Kay et al., 2007; Römer et al., 2007) with their N-terminal domain exhibiting general DNA-binding properties (Gao et al., 2012) and the repeat region facilitating specific interaction to DNA bases (Boch et al., 2009; Moscou & Bogdanove, 2009).


In xanthomonads

Yes, in many pathovars, but not necesssarily all strains within a pathovar.

In other plant pathogens/symbionts

Yes: Genes homologous to avrBs3 of Xanthomonas were detected in some strains of Ralstonia solanacearum biovars 3, 4 and 5 (Heuer et al., 2007), in endofungal strains of Burkholderia rhizoxinica (Lacker et al., 2011), and in unknown marine organisms. All these related proteins can bind DNA (de Lange et al., 2013; de Lange et al., 2014; de Lange et al., 2015).


Boch J, Scholze H, Schornack S, Landgraf A, Hahn S, Kay S, Lahaye T, Nickstadt A, Bonas U (2009). Breaking the code of DNA binding specificity of TAL-type III effectors. Science 326: 1509-1512. DOI: 10.1126/science.1178811

Bonas U, Stall, RE, Staskawicz B (1989). Genetic and structural characterization of the avirulence gene avrBs3 from Xanthomonas campestris pv. vesicatoria. Mol. Gen. Genet. 218: 127-136. DOI: 10.1007/BF00330575

Bonas U, Schulte R, Fenselau S, Minsavage GV, Staskawicz BJ, Stall RE (1991). Isolation of a gene cluster from Xanthomonas campestris pv. vesicatoria that determines pathogenicity and the hypersensitive response on pepper and tomato. Mol. Plant Microbe Interact 4: 81-88. DOI: 10.1094/MPMI-4-081

Bonas U, Van den Ackerveken G (1999). Gene-for-gene interactions: bacterial avirulence proteins specify plant disease resistance. Curr. Opin. Microbiol. 2: 94-98. DOI: 10.1016/S1369-5274(99)80016-2

de Lange O, Schreiber T, Schandry N, Radeck J, Braun KH, Koszinowski J, Heuer H, Strauß A, Lahaye T (2013). Breaking the DNA-binding code of Ralstonia solanacearum TAL effectors provides new possibilities to generate plant resistance genes against bacterial wilt disease. New Phytol. 199: 773-786. DOI: 10.1111/nph.12324

de Lange O, Wolf C, Dietze J, Elsaesser J, Morbitzer R, Lahaye T (2014). Programmable DNA-binding proteins from Burkholderia provide a fresh perspective on the TALE-like repeat domain. Nuc. Acids Res. 42: 7436-7449. DOI: 10.1093/nar/gku329.

de Lange O, Wolf C, Thiel P, Krüger J, Kleusch C, Kohlbacher O, Lahaye T (2015). DNA-binding proteins from marine bacteria expand the known sequence diversity of TALE-like repeats. Nuc. Acids Res. 43: 10065-10080. DOI: 10.1093/nar/gkv1053

Deng D, Yan C, Pan X, Mahfouz M, Wang J, Zhu JK, Shi Y, Yan N (2012a). Structural basis for sequence-specific recognition of DNA by TAL effectors. Science 335: 720-723. DOI: 10.1126/science.1215670

Deng D, Yan C, Wu J, Pan X, Yan N (2014). Revisiting the TALE repeat. Protein Cell 5: 297-306. DOI: 10.1007/s13238-014-0035-2

Deng D, Yin P, Yan C, Pan X, Gong X, Qi S, Xie T, Mahfouz M, Zhu JK, Yan N, Shi Y (2012b). Recognition of methylated DNA by TAL effectors. Cell Res. 22: 1502-1504. DOI: 10.1038/cr.2012.127

Gao H, Wu X, Chai J, Han Z (2012). Crystal structure of a TALE protein reveals an extended N-terminal DNA binding region. Cell Res. 22: 1716-1720. DOI: 10.1038/cr.2012.156

Herbers K, Conrads-Strauch J, Bonas U (1992). Race-specificity of plant resistance to bacterial spot disease determined by repetitive motifs in a bacterial avirulence protein. Nature 356: 172-174. DOI: 10.1038/356172a0

Heuer H, Yin YN, Xue QY, Smalla K, Guo JH (2007). Repeat domain diversity of avrBs3-like genes in Ralstonia solanacearum strains and association with host preferences in the field. Appl. Environ. Microbiol. 73: 4379-4384. DOI: 10.1128/AEM.00367-07

Hopkins CM, White FF, Choi SH, Guo A, Leach JE (1992). Identification of a family of avirulence genes from Xanthomonas oryzae pv. oryzae. Mol. Plant Microbe Interact. 5: 451-459. DOI: 10.1094/mpmi-5-451

Kay S, Hahn S, Marois E, Hause G, Bonas U (2007). A bacterial effector acts as a plant transcription factor and induces a cell size regulator. Science 318: 648-651. DOI: 10.1126/science.1144956

Knoop V, Staskawicz B, Bonas U (1991). Expression of the avirulence gene avrBs3 from Xanthomonas campestris pv. vesicatoria is not under the control of the hrp genes and is independent of plant factors. J. Bacteriol. 173: 7142-7150. DOI: 10.1128/jb.173.22.7142-7150.1991

Lackner G, Moebius N, Partida-Martinez LP, Boland S, Hertweck C (2011). Evolution of an endofungal lifestyle: Deductions from the Burkholderia rhizoxinica genome. BMC Genomics 12: 210. DOI: 10.1186/1471-2164-12-210

Mak AN, Bradley P, Cernadas RA, Bogdanove AJ, Stoddard BL (2012). The crystal structure of TAL effector PthXo1 bound to its DNA target. Science 335: 716-719. DOI: 10.1126/science.1216211

Marois E, Van den Ackerveken G, Bonas U (2002). The Xanthomonas type III effector protein AvrBs3 modulates plant gene expression and induces cell hypertrophy in the susceptible host. Mol. Plant Microbe Interact. 15: 637-646. DOI: 10.1094/MPMI.2002.15.7.637

Minsavage GV, Dahlbeck D, Whalen MC, Kearney B, Bonas U, Staskawicz BJ, Stall, RE (1990). Gene-for-gene relationships specifying disease resistance in Xanthomonas campestris pv. vesicatoria-pepper interactions. Mol. Plant Microbe Interact. 3: 41-47. DOI: 10.1094/MPMI-3-041

Moscou MJ, Bogdanove AJ (2009). A simple cipher governs DNA recognition by TAL effectors. Science 326: 1501. DOI: 10.1126/science.1178817

Murakami MT, Sforça ML, Neves JL, Paiva JH, Domingues MN, Pereira AL, Zeri AC, Benedetti CE (2010). The repeat domain of the type III effector protein PthA shows a TPR-like structure and undergoes conformational changes upon DNA interaction. Proteins 78: 3386-3395. DOI: 10.1002/prot.22846

Römer P, Hahn S, Jordan T, Strauss T, Bonas U, Lahaye T (2007). Plant pathogen recognition mediated by promoter activation of the pepper Bs3 resistance gene. Science 318: 645-648. DOI: 10.1126/science.1144958

Rossier O, Wengelnik K, Hahn K, Bonas U (1999). The Xanthomonas Hrp type III system secretes proteins from plant and mammalian bacterial pathogens. Proc. Natl. Acad. Sci. USA 96: 9368-9373. DOI: 10.1073/pnas.96.16.9368

Scheibner F, Marillonnet S, Büttner D (2017). The TAL effector AvrBs3 from Xanthomonas campestris pv. vesicatoria contains multiple export signals and can enter plant cells in the absence of the type III secretion translocon. Front Microbiol. 8: 2180. DOI: 10.3389/fmicb.2017.02180

Stella S, Molina R, Yefimenko I, Prieto J, Silva G, Bertonati C, Juillerat A, Duchateau P, Montoya G (2013). Structure of the AvrBs3–DNA complex provides new insights into the initial thymine-recognition mechanism. Acta Cryst. 69: 1707-1716. DOI: 10.1107/S0907444913016429

Szurek B, Marois E, Bonas U, Van den Ackerveken G (2001). Eukaryotic features of the Xanthomonas type III effector AvrBs3: protein domains involved in transcriptional activation and the interaction with nuclear import receptors from pepper. Plant J. 26: 523-534. DOI: 10.1046/j.0960-7412.2001.01046.x

Szurek B, Rossier O, Hause G, Bonas U (2002). Type III-dependent translocation of the Xanthomonas AvrBs3 protein into the plant cell. Mol. Microbiol. 46: 13-23. DOI: 10.1046/j.1365-2958.2002.03139.x

Van den Ackerveken G, Marois E, Bonas U (1996). Recognition of the bacterial avirulence protein AvrBs3 occurs inside the host plant cell. Cell 87: 1307-1316. DOI: 10.1016/S0092-8674(00)81825-5

Yin P, Deng D, Yan C, Pan X, Xi JJ, Yan N, Shi Y (2012). Specific DNA-RNA hybrid recognition by TAL effectors. Cell Rep. 2: 707-713. DOI: 10.1016/j.celrep.2012.09.001

Further reading

Boch J, Bonas U (2010). Xanthomonas AvrBs3 family-type III effectors: discovery and function. Annu. Rev Phytopathol. 48: 419-436. DOI: 10.1146/annurev-phyto-080508-081936

Boch J, Bonas U, Lahaye T (2014). TAL effectors - pathogen strategies and plant resistance engineering. New Phytol. 204: 823-832. DOI: 10.1111/nph.13015

Bogdanove AJ, Schornack S, Lahaye T (2010). TAL effectors: finding plant genes for disease and defense. Curr. Opin. Plant Biol. 13: 394-401. DOI: 10.1016/j.pbi.2010.04.010

Doyle EL, Stoddard BL, Voytas DF, Bogdanove AJ (2013). TAL effectors: highly adaptable phytobacterial virulence factors and readily engineered DNA-targeting proteins. Trends Cell Biol. 23: 390-398. DOI: 10.1016/j.tcb.2013.04.003

Hutin M, Pérez-Quintero AL, Lopez C, Szurek B (2015). MorTAL Kombat: the story of defense against TAL effectors through loss-of-susceptibility. Front. Plant Sci. 6: 535. DOI: 10.3389/fpls.2015.00535. Erratum in: Front Plant Sci. (2015) 6: 647.

Xue J, Lu Z, Liu W, Wang S, Lu D, Wang X, He X (2020). The genetic arms race between plant and Xanthomonas: lessons learned from TALE biology. Sci. China Life Sci. 63. DOI: 10.1007/s11427-020-1699-4

Zhang J, Yin Z, White F (2015). TAL effectors and the executor R genes. Front. Plant Sci. 6: 641. DOI: 10.3389/fpls.2015.00641

bacteria/t3e/avrbs3.txt · Last modified: 2020/11/30 13:33 by st