AvrXa10 Contains an Acidic Transcriptional Activation Domain in the Functionally Conserved C Terminus

The avrXa10 gene of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial blight of rice, is a member of the avrBs3 avirulence gene family and directs the elicitation of resistance in a gene-for-gene manner on rice lines carrying the resistance gene Xa10. The carboxyl (C) terminus of AvrXa10 has a previously undescribed domain that is structurally similar to the acidic activation domain of many eukaryotic transcription factors in addition to three nuclear localization signal (NLS) sequences. Removal of the C-terminal 38 codons containing the putative activation domain, but retaining the NLS sequences, was concomitant with the loss of avirulence activity. The C-terminal coding regions of avrBs3 and avrXa7 can be replaced by the corresponding region of avrXa10, and the genes retained specificity for the resistance genes Bs3 in pepper and Xa7 in rice, respectively. The avrBs3 and avrXa7 avirulence activities of the hybrid genes were also lost upon removal of the terminal 38 codons. When fused to the coding sequence of the Gal4 DNA binding domain, AvrXa10 activated transcription in yeast and Arabidopsis thaliana. Removal of the carboxyl region severely reduced transcriptional activation. AvrXa10 would have to be localized to the host cell nucleus to function autonomously in transcriptional activation. Consistent with this requirement, mutations in all three NLS sequences of avrXa10 caused a loss in avirulence activity. The findings demonstrate the requirement of the C terminus for AvrXa10 function and the potential for the members of this family of avirulence gene products to enter the host nucleus and alter host transcription.