The soil-borne pathogen Agrobacterium tumefaciens is a remarkably clever little microbe. After entering a plant through a wound site, the bacterium inserts a segment of its own DNA into the plant's genome. The genes in this insertion stimulate the synthesis of plant growth hormones, which cause a tumerous proliferation of plant cells that leads to a blight known as crown gall disease. And those genes instruct the plant to produce opines--compounds containing the carbon and nitrogen Agrobacterium lives on.
This example of genetic transfer between different kingdoms of organisms--from animals to plants--is the only one known. (Viruses also transfer DNA to plants, but because they can not survive outside of a host cell, they are not themselves generally considered to be living organisms.) Yet despite close scrutiny, scientists have been unable to determine the exact mechanism by which the bacterial DNA crosses the plant's so-called plasma membrane. The results of research published yesterday in the Proceedings of the National Academy of Sciences, however, offer new insight. According to the report, a single protein dubbed VirE2 may serve as the plasma membrane gatekeeper that allows the Agrobacterium DNA segment to pass through.
Fabrice Dumas of Biozentrum in Basel, Switzerland and Myriam Duckely of the Friedrich Miescher Institute (also in Basel) and their colleagues demonstrated through a series of experiments that VirE2 interacts with lipids (important components of membranes), and forms large channels or pores in artificial, plant plasma-like membranes, depending on the electric potential across that membrane. These channels appear to selectively admit negatively charged molecules and single-stranded DNA, which is exactly what Agrobacterium needs to send in. Such specificity for DNA transport sets VirE2 apart from other protein gatekeepers. And as such, the authors report, it may serve as an important tool in the development of new systems for drug delivery and gene therapy.