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Characterization of Sec-translocon dependent extracytoplasmic proteins and essential signal peptidase I of Candidatus Liberibacter asiaticus
S. PRASAD (1), S. Prasad (1), N. Wang (1). (1) University of Florida, Lake Alfred, FL, U.S.A.

Citrus Greening is a destructive disease in citrus plants globally. A comprehensive view of the molecular interactions of the causative agent, <i>Candidatus</i> Liberibacter asiaticus (Las) with its plant and vector hosts (citrus and <i>Diaphorina citri </i>respectively) has not yet been established, owing to constraints in maintenance of a pure culture of the bacterium. Interestingly, Las contains a complete Sec-translocon, a potent system for the transportation of Las proteins including the virulence factors into the extracytoplasmic milieu. We characterize the Sec-translocon dependent, signal peptide (SP) containing extracytoplasmic proteins of Las. A total of 150 Las proteins were predicted to contain SP targeting them out of the cytoplasm using four dedicated online algorithms. Experimental validation of the predictions was conducted with <i>E. coli</i> based <i>phoA</i> gene fusion assays. The study revealed 89 proteins to be extracytoplasmic in the annotated Las proteome. Further comparison analyses of these 89 proteins of Las were carried out with <i>Liberibacter crescens</i> BT-1. Additionally, we assessed the function of Las signal peptidase I (<i>lep</i>) responsible for cleaving the SP of most exported proteins, the Las lep restored the growth of otherwise conditionally lethal amber mutants of <i>lep</i> gene in <i>E. coli</i>. This is the first study describing the Sec-translocon dependent extracytoplasmic proteome of Las and will contribute in the identification of Sec-translocon dependent effector proteins

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