Abstract
LdtR is a master regulator of gene expression in Liberibacter asiaticus, one of the causative agents of citrus greening disease. LdtR belongs to the MarR-family of transcriptional regulators and it has been linked to the regulation of more than 180 genes in Liberibacter species, most of them gathered in the following Clusters of Orthologous Groups: cell motility, cell wall envelope, energy production, and transcription. Our previous transcriptomic evidence suggested that LdtR is directly involved in the modulation of the zinc uptake system genes (znu) in the closely related L. crescens. In this report, we show that LdtR is involved in the regulation of one of the two encoded zinc uptake mechanisms in L. asiaticus, named znu2. We also show that LdtR binds zinc with higher affinity than benzbromarone, a synthetic effector inhibitory molecule, resulting in the disruption of the LdtR:promoter interactions. Using site-directed mutagenesis, electrophoretic mobility shift assays (EMSAs), and isothermal titration calorimetry, we identified that residues C28 and T43 in LdtR, located in close proximity to the Benz1 pocket, are involved in the interaction with zinc. These results provided new evidence of a high-affinity effector molecule targeting a key player in L. asiaticus’ physiology and complemented our previous findings about the mechanisms of signal transduction in members of the MarR-family.
Highlights
Huanglongbing (HLB) or citrus greening disease is predominantly caused by Liberibacter asiaticus, a Gram-negative, phloem-limited, fastidious α-proteobacterium
LdtR belongs to the MarR-family of transcriptional regulators and it has been linked to the regulation of more than 180 genes in Liberibacter species, most of them gathered in the following Clusters of Orthologous Groups: cell motility, cell wall envelope, energy production, and transcription
The genome of L. asiaticus psy62 encodes for two gene clusters with homology to the znuABC system described in E. coli and Sinorhizobium meliloti [24,32]
Summary
Huanglongbing (HLB) or citrus greening disease is predominantly caused by Liberibacter asiaticus, a Gram-negative, phloem-limited, fastidious α-proteobacterium. This bacterium is transmitted from plant to plant by the phloem-sap feeder psyllid named Diaphorina citri Kuwayama [1] and has generated a devastation in the citrus industry over the last 10 years [1,2,3]. Among the symptoms usually observed in the plant are yellow blotchy mottle, asymmetrical leaf chlorosis, premature fruit drop, as well as reduced fruit and juice quality [4,5,6] Some of these distinguishing symptoms are associated with the limitation of nutrients [1,7].
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