Regulation of Bacterial Type IV Secretion

This chapter explores the question of how bacterial pathogens regulate the biogenesis and function of their type IV secretion systems (T4SSs) in pathogenic settings. First, it describes a regulatory cascade involving the perception of multiple signals exchanged between Agrobacterium tumefaciens and its plant host. This signaling dialogue leads not only to infection of plant tissue but also to enhanced conjugative transfer of the virulence-associated tumor-inducing (pTi) plasmid. Second, it summarizes the regulatory features of a large T4SS subfamily, the conjugation systems functioning in gram-negative and -positive species. The chapter then summarizes regulatory features of the well-characterized effector translocators of Brucella spp., Legionella pneumophila, and Bartonella spp., and also examines why and how these and other bacterial pathogens cross-regulate T4SSs and other surface motility or attachment devices such as flagella and type IV pili. Finally, the chapter discusses post-transcriptional regulation of substrate-T4SS docking reactions and donor-target cell contacts. The overarching goal of this chapter is to identify mechanistic themes and variations that have evolved to regulate the myriad of T4SS activities exploited by bacterial pathogens during infection. The conjugation systems are the largest subfamily, present in nearly all bacterial species and some archaeal species. Many bacterial pathogens rely on flagellar or type IV pilus-based motility to migrate to sites favorable for colonization within the host. For all T4SSs, transduction of exogenous or physiological signals ultimately converges on the regulatory machinery controlling transcription of machine subunits, DNA processing enzymes, or protein effectors.