Identification of flavonoids as regulators of YbeY activity in Liberibacter asiaticus

Summary Liberibacter asiaticus is the prevalent causative pathogen of Huanglongbing or citrus greening disease, which has resulted in a devastating crisis in the citrus industry. A thorough understanding of this pathogen's physiology and mechanisms to control cell survival is critical in the identification of therapeutic targets. YbeY is a highly conserved bacterial RNase that has been implicated in multiple roles. In this study, we evaluated the biochemical characteristics of the L . asiaticus YbeY (CLIBASIA_01560) and assessed its potential as a target for antimicrobials. YbeY Las was characterized as an endoribonuclease with activity on 3′ and 5′ termini of 16S and 23S rRNAs, and the capacity to suppress the E . coli ΔybeY phenotype. We predicted the YbeY Las protein:ligand interface and subsequently identified a flavone compound, luteolin, as a selective inhibitor. Site‐directed mutagenesis was subsequently used to identify key residues involved in the catalytic activity of YbeY Las . Further evaluation of naturally occurring flavonoids in citrus trees indicated that both flavones and flavonols had potent inhibitory effects on YbeY Las . Luteolin was subsequently examined for efficacy against L . asiaticus in Huanglongbing‐infected citrus trees, where a significant reduction in L . asiaticus gene expression was observed.