Erwinia carotovoraSubspecies Produce Duplicate Variants of ExpR, LuxR Homologs That ActivatersmATranscription but Differ in Their Interactions withN-Acylhomoserine Lactone Signals

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Abstract
TheN-acylhomoserine lactone (AHL) signaling system comprises a producing system that includes acylhomoserine synthase (AhlI, a LuxI homolog) and a receptor, generally a LuxR homolog. AHL controls exoprotein production inErwinia carotovoraand consequently the virulence for plants. In previous studies we showed that ExpR, a LuxR homolog, is an AHL receptor and that it activates transcription ofrsmA, the gene encoding an RNA binding protein which is a global negative regulator of exoproteins and secondary metabolites. An unusual finding was that the transcriptional activity of ExpR was neutralized by AHL. We subsequently determined that the genomes of most strains ofE. carotovorasubspecies tested possess two copies of theexpRgene:expR1, which was previously studied, andexpR2, which was the focus of this study. Comparative analysis of the two ExpR variants ofE. carotovorasubsp.carotovorashowed that while both variants activatedrsmAtranscription, there were significant differences in the patterns of their AHL interactions, thersmAsequences to which they bound, and their relative efficiencies of activation ofrsmAtranscription. An ExpR2−mutant produced high levels of exoproteins and reduced levels of RsmA in the absence of AHL. This contrasts with the almost complete inhibition of exoprotein production and the high levels of RsmA production in an AhlI−mutant that was ExpR1−. Our results suggest that ExpR2 activity is responsible for regulating exoprotein production primarily by modulating the levels of an RNA binding protein.

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