A TALE of Transposition: Tn 3 -Like Transposons Play a Major Role in the Spread of Pathogenicity Determinants of Xanthomonas citri and Other Xanthomonads

Abstract

ABSTRACT Members of the genus Xanthomonas are among the most important phytopathogens. A key feature of Xanthomonas pathogenesis is the translocation of type III secretion system (T3SS) effector proteins (T3SEs) into the plant target cells via a T3SS. Several T3SEs and a murein lytic transglycosylase gene ( mlt , required for citrus canker symptoms) are found associated with three transposition-related genes in Xanthomonas citri plasmid pXAC64. These are flanked by short i nverted r epeats (IRs). The region was identified as a transposon, Tn Xax1 , with typical Tn 3 family features, including a transposase and two recombination genes. Two 14-bp palindromic sequences within a 193-bp potential resolution site occur between the recombination genes. Additional derivatives carrying different T3SEs and other passenger genes occur in different Xanthomonas species. The T3SEs include t ranscription a ctivator- l ike e ffectors (TALEs). Certain TALEs are flanked by the same IRs as found in Tn Xax1 to form m obile i nsertion c assettes (MICs), suggesting that they may be transmitted horizontally. A significant number of MICs carrying other passenger genes (including a number of TALE genes) were also identified, flanked by the same Tn Xax1 IRs and delimited by 5-bp target site duplications. We conclude that a large fraction of T3SEs, including individual TALEs and potential pathogenicity determinants, have spread by transposition and that Tn Xax1 , which exhibits all of the essential characteristics of a functional transposon, may be involved in driving MIC transposition. We also propose that TALE genes may diversify by fork slippage during the replicative Tn 3 family transposition. These mechanisms may play a crucial role in the emergence of Xanthomonas pathogenicity. IMPORTANCE Xanthomonas genomes carry many i nsertion s equences (IS) and transposons, which play an important role in their evolution and architecture. This study reveals a key relationship between transposons and pathogenicity determinants in Xanthomonas. We propose that several transposition events mediated by a Tn 3 -like element carrying different sets of passenger genes, such as different type III secretion system effectors (including t ranscription a ctivation- l ike e ffectors [TALEs]), were determinant in the evolution and emergence of Xanthomonas pathogenicity. TALE genes are DNA-binding effectors that modulate plant transcription. We also present a model for generating TALE gene diversity based on fork slippage associated with the replicative transposition mechanism of Tn 3 -like transposons. This may provide a mechanism for niche adaptation, specialization, host-switching, and other lifestyle changes. These results will also certainly lead to novel insights into the evolution and emergence of the various diseases caused by different Xanthomonas species and pathovars.