Phytobacterial Type VI Secretion System - Gene Distribution, Phylogeny, Structure and Biological Functions

Abstract

Microbes, and their distant relatives, plants, are thought to have co-evolved during the last 2 billion years. Most of the plant-associated prokaryotes are commensals, found primarily on leaf surfaces or roots, and have no discernible or known effects on plant growth or physiology; others evolved more or less intimate relationships with plants such as N-fixing symbioses, endophytic existence or plant growth-promoting (rhizobacterial) associations; yet others, the minority, wage outright hostility with plants, inciting various diseases. Although some phytopathogenic bacteria internalize themselves in the plant vascular system, most of them colonize plant tissues extracellularly and target plant cell wall and membrane or internal cellular structures, signaling systems and metabolic machinery from the outside. For targeting they deploy phytotoxic metabolites, hormones, polysaccharides, enzymes for the hydrolysis of cell walls and other catalytic macromolecular effectors (and, exceptionally, DNA) as “ballistic missiles”. To accomplish efficient transport of macromolecules across the bacterial and/or the plant cell envelop (plant cell wall and membrane), Gram-negative bacteria possess a suite of specialized transport systems, dedicated to the transport of selected sets of proteins from the bacterial cytoplasm to the external environment or into other living cells. Type I to type VI secretion systems (abbreviated T1SS to T6SS) form channels by assembling oligomeric macromolecular complexes of varying composition and sophistication. These assemblies function as molecular machines, are broadly conserved across Gram-negative bacteria and play important roles in the virulence of pathogens. In general, most of these systems require a component providing energy to the secretion process (usually an ATPase), an outer-membrane protein, and various components involved either in scaffolding the macromolecular complex into the cell envelope or in the specific recognition of secreted substrates. It is noteworthy that certain components of these secretion machines are thought to be derived from other membrane-bound multiprotein structures serving a different purpose (see below).