Molecular analysis of sucrose metabolism of Erwinia amylovora and influence on bacterial virulence.

Sucrose is an important storage and transport sugar of plants and an energy source for many phytopathogenic bacteria. To analyze regulation and biochemistry of sucrose metabolism of the fire blight pathogen Erwinia amylovora, a chromosomal fragment which enabled Escherichia coli to utilize sucrose as sole carbon source was cloned. By transposon mutagenesis, the scr regulon of E. amylovora was tagged, and its nucleotide sequence was determined. Five open reading frames, with the genes scrK, scrY, scrA, scrB, and scrR, had high homology to genes of the scr regulons from Klebsiella pneumoniae and plasmid pUR400. scrB and scrR of E. amylovora were fused to a histidine tag and to the maltose-binding protein (MalE) of E. coli, respectively. ScrB (53 kDa) catalyzed the hydrolysis of sucrose with a K(m) of 125 mM. Binding of a MalE-ScrR fusion protein to an scrYAB promoter fragment was shown by gel mobility shifts. This complex dissociated in the presence of fructose but not after addition of sucrose. Expression of the scr regulon was studied with an scrYAB promoter-green fluorescent protein gene fusion and measured by flow cytometry and spectrofluorometry. The operon was affected by catabolite repression and induced by sucrose or fructose. The level of gene induction correlated to the sucrose concentration in plant tissue, as shown by flow cytometry. Sucrose mutants created by site-directed mutagenesis did not produce significant fire blight symptoms on apple seedlings, indicating the importance of sucrose metabolism for colonization of host plants by E. amylovora.