MOLECULAR ANALYSIS OF AVIRULENCE GENE AVRRPT2 AND IDENTIFICATION OF APUTATIVE REGULATORY SEQUENCE COMMON TO ALL KNOWN PSEUDOMONAS-SYRINGAEAVIRULENCE GENES

The avrRpt2 locus from Pseudomonas syringae pv. tomato causes virulent strains of P. syringae to be avirulent on some, but not all, lines of Arabidopsis thaliana and Glycine max (soybean). We determined the DNA sequence of the avrRpt2 locus and identified the avrRpt2 gene as a 76 8-bp open reading frame encoding a putative 28.2-kDa protein. Deletion analysis and transcription studies provided further evidence that thi s open reading frame encodes AvrRpt2. We found that the avrRpt2 gene a lso has avirulence activity in P. syringae pathogens of Phaseolus vulg aris (common bean), suggesting that disease resistance genes specific to avrRpt2 are functionally conserved among diverse plant species. The predicted AvrRpt2 protein is hydrophilic and contains no obvious memb rane-spanning domains or export signal sequences, and there was no sig nificant similarity of AvrRpt2 to sequences in the GenBank, EMBL, or S wiss PIR data bases. A comparison of the avrRpt2 DNA sequence to nine other P. syringae avirulence genes revealed a highly conserved sequenc e, GGAACCNA-N14-CCACNNA, upstream of the translation initiation codon. This motif is located 6 to 8 nucleotides upstream of the transcriptio n start site in all four P. syringae avirulence genes for which a tran scription start site has been determined, suggesting a role as a bindi ng site for a novel form of RNA polymerase. Regulation of avrRpt2 was similar to other P. syringae avirulence genes; expression was high in minimal medium and low in rich medium and depended on the hrpRS locus and an additional locus at the opposite end of the hrp cluster of P. s yringae pv. tomato.