Ring around the virus: RNA packs in the DNA.

The next time you struggle to stuff your clothes into a suitcase, you may well envy the skill of viruses that infect bacteria. Known as bacteriophages, these viruses create copies of themselves by building shells of protein (SN: 7/18/98, p. 38) and then cramming their DNA inside. "If the protein shell [were] an inch in diameter, the DNA would be 14 feet long. It gets stuffed in there in a few minutes. It's amazing," marvels Dwight Anderson of the University of Minnesota in Minneapolis.

Research groups led by Anderson and by his former student, Peixuan Guo of Purdue University in West Lafayette, Ind., have now deduced the unusual shape of a key packing tool employed by some, perhaps all, bacteriophages. The instrument is a ring formed by six identical strands of ribonucleic acid, or RNA, the two groups report independently in the July Molecular Cell.

While scientists know that single strands of RNA can store information or act as enzymes, they hadn't realized that they could form so complex a structure. "The circular hexamer ... implied by these experiments is something new and exciting in the world of RNA structure," says Roger W. Hendrix of the University of Pittsburgh in a commentary in the July 24 Cell.

"It's the first instance in which identical subunits of RNA actually build a structure--one that does something very interesting: move DNA from the outside to the inside of a shell," says Anderson.

About a decade ago, Guo discovered that the bacteriophage phi-29 employs a short strand of RNA, dubbed packaging RNA (pRNA), to fill its empty shells with DNA. Researchers later showed that pRNA binds to a shell's connector, the ring of proteins encircling the hole in the shell where DNA goes in during packaging and comes out when the virus infects a bacterium.

While studies have indicated that phi-29 requires multiple strands of pRNA, Anderson's and Guo's groups recently found that six strands joined into a ring are both sufficient and necessary for DNA packaging. Guo's team also identified specific parts of the pRNA strand that enable it to combine with other pRNA strands.

Although this latest research doesn't resolve how phi-29's DNA packaging machinery works, researchers believe that knowing the structures of its crucial components will be invaluable. "Once you get a look at the whole machine, you can have some ideas about how it works," says Anderson.

Hendrix, for example, has theorized that a bacteriophage's DNA threads into its shell as the connector rotates like a nut around a bolt. The ring of pRNA, speculates Guo, may bring about this rotation by contracting and elongating its six strands sequentially.