The Xanthomonas citri pv. citri type vi secretion system: function, gene regulation and secreted substrates

Abstract

Bacterial type VI secretion systems (T6SS) are protein complexes that span the bacterial cell envelope and are involved in protein translocation to target cells. It was not until recently that T6SS were first described and structural and functional studies indicate that they act by a mechanism similar to injection of DNA into bacterial cells by bacteriophages, specially the T4 phage. T6SS are essential for virulence in diverse bacteria and are also involved in interbacterial competition, secreting toxins that target neighbor cells, increasing the fitness of T6SS-containing cells. Xanthomonas citri pv citri (Xac) is the causative agent of citrus canker on a wide variety of citrus cultivars, resulting in significant losses to the agriculture in Brazil. The Xac genome encodes a complete T6SS, which is split into two clusters, separated by a region containing 10 genes. This project will use bacterial genetics and molecular biology approaches to functionally characterize Xac T6SS. For that, phenotypes of mutant strains in T6SS genes will be characterized. Also, expression of T6SS components will be analyzed under distinct growth conditions and in planta. To identify T6SS protein substrates, the secretome of strains with altered activity of T6SS will be analyzed. Two alternative sigma factors encoded in the region flanked by the T6SS clusters will be characterized by transcriptome analysis of strains expressing high levels of these regulators and phenotypic characterization of mutant strains. This work will provide new insights in the biology of T6SS function and regulation and also molecular mechanisms of Xac physiology and pathogenicity. (AU)