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A LuxR Homolog in a Cottonwood Tree Endophyte That Activates Gene Expression in Response to a Plant Signal or Specific Peptides.
Monday, 2016/08/08 | 07:56:46

Schaefer AL, Oda Y, Coutinho BG, Pelletier DA, Weiburg J, Venturi V, Greenberg EP, Harwood CS.

MBio. 2016 Aug 2;7(4). pii: e01101-16. doi: 10.1128/mBio.01101-16.

Abstract

Homologs of the LuxR acyl-homoserine lactone (AHL) quorum-sensing signal receptor are prevalent in Proteobacteria isolated from roots of the Eastern cottonwood tree, Populus deltoides. Many of these isolates possess an orphan LuxR homolog, closely related to OryR from the rice pathogen Xanthomonas oryzae OryR does not respond to AHL signals but, instead, responds to an unknown plant compound. We discovered an OryR homolog, PipR, in the cottonwood endophyte Pseudomonas sp. strain GM79. The genes adjacent to pipR encode a predicted ATP-binding cassette (ABC) peptide transporter and peptidases. We purified the putative peptidases, PipA and AapA, and confirmed their predicted activities. A transcriptional pipA-gfp reporter was responsive to PipR in the presence of plant leaf macerates, but it was not influenced by AHLs, similar to findings with OryR. We found that PipR also responded to protein hydrolysates to activate pipA-gfp expression. Among many peptides tested, the tripeptide Ser-His-Ser showed inducer activity but at relatively high concentrations. An ABC peptide transporter mutant failed to respond to leaf macerates, peptone, or Ser-His-Ser, while peptidase mutants expressed higher-than-wild-type levels of pipA-gfp in response to any of these signals. Our studies are consistent with a model where active transport of a peptidelike signal is required for the signal to interact with PipR, which then activates peptidase gene expression. The identification of a peptide ligand for PipR sets the stage to identify plant-derived signals for the OryR family of orphan LuxR proteins.

IMPORTANCE:

We describe the transcription factor PipR from a Pseudomonas strain isolated as a cottonwood tree endophyte. PipR is a member of the LuxR family of transcriptional factors. LuxR family members are generally thought of as quorum-sensing signal receptors, but PipR is one of an emerging subfamily of LuxR family members that respond to compounds produced by plants. We found that PipR responds to a peptidelike compound, and we present a model for Pip system signal transduction. A better understanding of plant-responsive LuxR homologs and the compounds to which they respond is of general importance, as they occur in dozens of bacterial species that are associated with economically important plants and, as we report here, they also occur in members of certain root endophyte communities.

 

See http://www.ncbi.nlm.nih.gov/pubmed/27486195

 

FIG 1 

Pseudomonas sp. GM79 genomic region surrounding the oryR homolog pipR (red, PMI36_04623). The region includes genes predicted to encode peptidases (yellow, PMI36_04622 and PMI36_04624) and an ABC-type peptide transporter (blue, PMI36_04617-04621). There are five peptide transporter genes coding for one periplasmic binding protein, two nucleotide-binding domain (NBD) proteins, and two transmembrane domain (TMD) proteins. The positive numbers below the genes indicate the number of bases in the intergenic region separating the genes; a negative number indicates there is overlap of the two genes.

 

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