Putative Transcriptional Regulator LysR1 Is Required for Full Virulence of Erwinia amylovora

Fire blight, caused by the gram-negative bacterium Erwinia amylovora, is a serious disease of apple and pear trees. Here, we report genetic evidence that the predicted transcription factor LysR1 is required for full E. amylovora virulence. A transposon insertion mutant and an allelic exchange mutant of lysR1 exhibited reduced virulence compared to the wild type in host apple tissues and a plasmid-borne copy of lysR1 restored virulence to the mutants. Unexpectedly, the lysR1 mutants were not auxotrophic for lysine. Numerous virulence factors, such as type III secretion system function, swimming motility and lipopolysaccharide and levansucrase enzyme production, were unchanged in lysR1 mutants compared to the wild type. However, lysR1 mutants were specifically deficient in production of the exopolysaccharide amylovoran, an essential E. amylovora virulence factor. Global gene expression pattern analysis using RNA sequencing revealed 34 differentially expressed genes between a lysR1 mutant and the wild type growing in culture. Expression of lysA, the presumptive LysR1 regulatory target, was reduced but not abolished in the lysR1 mutant. No expression differences were detected in genes involved in amylovoran biosynthesis or regulation, and none of the other differentially expressed genes had obvious links to E. amylovora virulence. The decreased amylovoran production in lysR1 mutants is presumably an indirect effect of loss of LysR1 activity, because LysR1 does not appear to be required for the expression of amylovoran biosynthesis genes. Nonetheless, amylovoran deficiency most likely contributes to the virulence defect of lysR1 mutants and could be the major factor leading to the virulence reduction.