Genetics of sorbitol metabolism in Erwinia amylovora and its influence on bacterial virulence.

A chromosomal DNA fragment from Erwinia amylovora was identified that complemented a deletion mutant in the gut(srl) operon of Escherichia coli. The E. amylovora srl operon on the cloned fragment was localized by transposon mutagenesis. A DNA fragment including the srl genes of E. amylovora was sequenced and found to contain six open reading frames (ORFs). These ORFs were highly homologous to genes of the gut operon of E. coli. No large gene was found that encoded a protein equivalent to GutA of E. coli; instead two ORFs with extensive similarity to GutA were identified in the E. amylovora srl operon. All transposon insertions were mapped by PCR analysis, and several insertions in a plasmid bearing the srl operon were unable to complement a mutation in the E. coli gutD gene. All E. amylovora srl mutants could be complemented by introducing the sorbitol operon from E. coli. The direction of transcription was confirmed by analysis of lacZ fusions. Expression of the srl operon in E. amylovora was high in the presence of sorbitol in the medium and was repressed by glucose. Mutants with a sorbitol deficiency were still virulent on slices of immature pears, but were unable to cause significant fire blight symptoms on apple shoots. Since sorbitol is used for carbohydrate transport in host plants of E. amylovora, this sugar alcohol may be an important factor in determining host specificity for the fire blight pathogen.