A deeper look into the pathogen responsible for crown gall disease in plants

The bacterium is parasitic: It infects its plant host by entering through an open wound, inserts a small segment of its genetic code into the plant's genome, devours energy made by the plant, and forms knobby brown lesions on the plant stem.

Huang's group focused on the pathogen's genetic material. Most bacteria have circular chromosomes. But A. tumefaciens C58, the strain studied by Huang's group, contains one circular chromosome and one linear chromosome (along with two circular plasmids). Huang's research illuminates how this bacterium maintains its linear chromosome.

Huang's team ascertained the DNA sequence for the telomeres, or the protective end caps, of the linear chromosome in A. tumefaciens C58 and confirmed that an enzyme, TelA, actually forms them by making hairpin loops. These end caps are important for maintaining the stability of linear chromosomes. Interestingly, TelA also binds the telomeres. This activity is unique among bacterial enzymes of this kind and may protect the telomeres (which degrade over time and thus lose their ability to preserve DNA), as telomere binding proteins do in eukaryotes.

"Hairpin-ended linear chromosomes and plasmids are found in a number of branches of bacteria and viruses," Huang says. "They are simple and elegant to form and to maintain." But what remains to be understood is why this linear configuration is not more common or even the preferred configuration for bacteria, Huang emphasizes.