FLAVOHAEMOGLOBIN HMPX - A NEW PATHOGENICITY DETERMINANT IN ERWINIA-CHRYSANTHEMI STRAIN-3937

Unlike wild-type Erwinia chrysanthemi strain 3937, which fully macerat es inoculated Saintpaulia plants, HmpX(-) mutants produce necrotic les ions or no symptoms. The hmpX gene was sequenced and the corresponding protein sequence analysed. We show that HmpX belongs to a family of f lavohaemoproteins (HMP), previously identified in two yeasts and in Es cherichia coli. Comparisons of protein sequences at the secondary stru cture level by hydrophobic cluster analysis have shown that HmpX posse sses two functional regions, a haemoglobin domain in its N-ternainal p art and a flavin reductase domain in its C-terminal part. In an HmpX(- ) strain, the synthesis of pectate lyases, which are pathogenicity det erminants in E. chrysanthemi, was reduced in conditions of low oxygen tension. Using gus fusion in hmpX, it was shown that hmpX transcriptio n was induced in coculture with tobacco cells. A putative function for HmpX is discussed.