Membrane-associated DNA Transport Machines

DNA pumps play important roles in bacteria during cell division and during the transfer of genetic material by conjugation and transformation. The FtsK/SpoIIIE proteins carry out the translocation of double-stranded DNA to ensure complete chromosome segregation during cell division. In contrast, the complex molecular machines that mediate conjugation and genetic transformation drive the transport of single stranded DNA. The transformation machine also processes this internalized DNA and mediates its recombination with the resident chromosome during and after uptake, whereas the conjugation apparatus processes DNA before transfer. This article reviews these three types of DNA pumps, with attention to what is understood of their molecular mechanisms, their energetics and their cellular localizations.The transport of DNA across membranes by bacteria occurs during sporulation, during cytokinesis, directly from other cells and from the environment. This review addresses the question “how is the DNA polyanion transferred processively across the hydrophobic membrane barrier”?DNA transport must occur through water-filled channels, at least conceptually addressing the problem posed by the hydrophobic membrane. DNA transporters presumably use metabolic energy directly or a coupled-flow (symporter or antiporter) mechanism to drive DNA processively through the channel. It is possible that a Brownian ratchet mechanism, in which directionality is imposed on a diffusive process, also contributes to transport.In this article, we will consider several DNA transport systems. We will begin with the simplest one, namely the FtsK/SpoIIIE system that is involved in cell division and sporulation. We will then turn to the more complex, multiprotein DNA uptake systems that accomplish genetic transformation (the uptake of environmental DNA from the environment) and the conjugation systems of Gram-negative bacteria that mediate the unidirectional transfer of DNA between cells. In each case we will discuss the proteins involved, their actions and the sources of energy that drive transport. Space limitations prevent discussion of other relevant topics, such as DNA transport during bacteriophage infection and more than a brief reference to conjugation in Gram-positive bacteria.