Characterization of the stress response by alternative sigma factors in Xanthomonas citri

Abstract

Bacterial sigma factors are subunits of the RNA polymerase that confer specificity to promoter DNA sequences and trigger transcription initiation. The vegetative sigma factor is responsible for transcription of housekeeping genes, whereas alternative sigma factors recognize different promoter sequences and regulate specific subsets of genes in response to variations in environmental conditions, allowing for bacterial adaptation. Extracytoplasmic function sigma factors (ECF) are the most diverse family of alternative sigma factors in bacteria and have been implicated in the adaptation to stresses perceived by the extracytoplasmic compartment of the cell (membranes and periplasm). Members of the Xanthomonas genus of phytopathogenic bacteria typically contain several genes encoding ECF and their functions remain largely elusive. X. citri is the causative agent of citrus canker, a disease that greatly impairs crop productivity. The bacterium is found in association with plants, either in plant debris deposited in the soil or colonizing the surface of leaves as an epiphyte, causing disease upon entering the mesophyll during stomata openings or through wounds. In silico analysis revealed nine genes encoding representatives of the sigma ECF family in the X. citri genome and only two of them have been functionally characterized. This project aims to perform functional studies of the other seven ECF of X. citri, with a focus on evaluating their roles in pathogenicity and adaptation to stresses found by the bacterium during colonization of host plants. For that, we will initially characterize the phenotype of mutant strains in each of these genes during growth under distinct conditions and in contact with host plants. In addition, we will identify the regulon and the mechanisms that control the activity of the most promising sigma factors identified in the phenotypical analysis. Results from this study will contribute to the understanding of the regulatory mechanisms that promote dissemination and survival of Xanthomonas.