Comparative analysis of the XopD type III secretion (T3S) effector family in plant pathogenic bacteria

SUMMARY XopD is a type III effector protein that is required for Xanthomonas campestris pathovar vesicatoria (Xcv) growth in tomato. It is a modular protein consisting of an N‐terminal DNA‐binding domain, two ethylene‐responsive element binding factor‐associated amphiphilic repression (EAR) transcriptional repressor motifs and a C‐terminal small ubiquitin‐related modifier (SUMO) protease. In tomato, XopD functions as a transcriptional repressor, resulting in the suppression of defence responses at late stages of infection. A survey of available genome sequences for phytopathogenic bacteria revealed that XopD homologues are limited to species within three genera of Proteobacteria— Xanthomonas , Acidovorax and Pseudomonas . Although the EAR motif(s) and SUMO protease domain are conserved in all XopD‐like proteins, variation exists in the length and sequence identity of the N‐terminal domains. Comparative analysis of the DNA sequences surrounding xopD and xopD‐like genes led to revised annotation of the xopD gene. Edman degradation sequence analysis and functional complementation studies confirmed that the xopD gene from Xcv encodes a 760‐amino‐acid protein with a longer N‐terminal domain than previously predicted. None of the XopD‐like proteins studied complemented Xcv Δ xopD mutant phenotypes in tomato leaves, suggesting that the N‐terminus of XopD defines functional specificity. Xcv Δ xopD strains expressing chimeric fusion proteins containing the N‐terminus of XopD fused to the EAR motif(s) and SUMO protease domain of the XopD‐like protein from X. campestris pathovar campestris strain B100 were fully virulent in tomato, demonstrating that the N‐terminus of XopD controls specificity in tomato.