Molecular characterization and hrp dependence of the avirulence gene avrPto from Pseudomonas syringae pv. tomato...

The avrPto avirulence gene from Pseudomonas syringae pv. tomato (Pst) race 0 governs race-specific resistance to bacterial speck disease in tomato cultivars containing the Pto resistance gene. The avrPto gene encodes 0.7 and 0.75 kb mRNAs whose predicted translation product is a mostly hydrophilic 164 amino acid protein of 18.3 kD a that reveals no homology to protein sequences in GenBank or EMBL databases. Highest expression of avrPto in cell culture is observed in minimal media containing sugars and sugar alcohols as carbon sources and lowest expression in minimal media containing tricarboxylic acid intermediates and in complex media. Expression of avrPto in planta is induced within 1 h following infection of both resistant and susceptible tomato plants by Pst, and increases over the first 6 h. Transcription of avrPto requires the hrpSR pathogenicity functions, but is independent of other Pst hrp genes. A region of the avrPto promoter shows homology to hrp box sequences upstream of other P. syringae genes that require the hrpSR locus for expression, and both avirulence activity and avrPto mRNA accumulation are abolished by deletions extending into this region. The avrPto transcription start site maps 31 nucleotides downstream of the hrp box motif.