MOLECULAR CHARACTERIZATION AND HRP DEPENDENCE OF THE AVIRULENCE GENE AVRPRO 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 e ncodes 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 e xpression of avrPto in cell culture is observed in minimal media conta ining sugars and sugar alcohols as carbon sources and lowest expressio n in minimal media containing tricarboxylic acid intermediates and in complex media. Expression of avrPto in planta is induced within 1 h fo llowing infection of both resistant and suceptible tomato plants by Ps t, 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 sequ ences upstream of other P. syringae genes that require the hrp SR locu s for expression, and both avirulence activity and avrPto mRNA accumul ation are abolished by deletions extending into this region. The avrPt o transcription start site maps 31 nucleotides downstream of the hrp b ox motif.