Nucleotide Sequence and Properties of the hrmA Locus Associated with the Pseudomonas syringae pv. syringae 61 hrp Gene Cluster. Sunggi Heu. Department of Botany, University of Maryland, College Park 20742 U.S.A. Steven W. Hutcheson. Department of Botany, University of Maryland, College Park 20742 U.S.A. MPMI 6:553-564. Accepted 11 June 1993. Copyright 1993 The American Phytopathological Society.

The hrmA locus, isolated from Pseudomonas syringae pv. syringae 61, is essential for phenotypic expression of the P. s. pv. syringae 61 hrp cluster in Escherichia coli strains and enables bacteria carrying the hrp/hrm gene cluster to elicit the hypersensitive response (HR) associated with plant disease resistance. The phenotype of P. s. pv. syringae 61 hrmA mutants (pathogenicity⁺, delayed HR) was distinct from that of hrp mutants. The locus was localized to a 3.6-kb BamH1-EcoR1 fragment whose nucleotide sequence was determined. A single open reading frame was identified that encodes for a 41,457-Da protein of unknown biochemical function. Production of the deduced protein product was confirmed by using T7 RNA polymerase-directed expression of the locus and N-terminal sequence analysis of the isolated HrmA. The deduced protein product did not exhibit homology with any of the characterized avr genes or the hrpN product of Erwinia amylovora. Transcription was shown to initiate 37 nucleotides upstream of the translational start from an apparent σ⁷⁰ promoter. Two hrp genes were shown to act as positive transcriptional factors for hrmA expression. Expression of hrmA in P. syringae pv. glycinea race 4 did not exhibit the phenotypic properties of an avr gene or HrpN, but suggested that this locus may serve a regulatory function. A homolog to hrmA was present in strains of only three of the 23 P. syringae pathovars tested.