Eukaryotic features of the Xanthomonas type III effector AvrBs3: protein domains involved in transcriptional activation and the interaction with nuclear import receptors from pepper

The AvrBs3 protein of the phytopathogenic bacterium Xanthomonas campestris pv. vesicatoria is targeted to host-plant cells by the bacterial Hrp type I II secretion system. In pepper plants containing the Bs3 resistance gene, A vrBs3 induces the hypersensitive response (HR). AvrBs3 recognition is thoug ht to occur in the plant cell nucleus as HR induction is dependent on nucle ar localization signals (NLSs) and an acidic transcription activation domai n (AAD). In a search for AvrBs3-interacting pepper proteins using the yeast two-hybrid system, we have isolated eight different classes of cDNA insert s including two genes for importin alpha proteins. Importin alpha is part o f the nuclear import machinery and interacts with AvrBs3 through an NLS in the carboxy-terminus of the protein, both in yeast and in vitro. The mechan ism of AvrBs3 recognition was further studied by analysis of the C-terminal AAD. This putative transcription-activation domain was shown to be require d for AvrBs3 HR-inducing activity, and could be functionally replaced with the VP16 AAD from the Herpes simplex virus. Our data support the model in w hich the AvrBs3 effector localizes to the nucleus, where the Bs3-mediated s urveillance system of resistant plants detects AvrBs3 through its interfere nce with host gene transcription.