July 27, 1998 WEST LAFAYETTE, Ind. -- Long known as messengers, transcribers and translators for DNA, the hard-working family of RNA molecules may also serve as chauffeurs. Peixuan Guo, professor of molecular virology at Purdue University, has found that a virus known as Bacteriophage Phi 29 uses six RNAs strung together in the shape of a hexagon to create a motor that transports DNA in the virus.
His findings were published in the July issue of the scientific journal Molecular Cell . A review covering this study also appears in the July 24 issue of Cell .
Guo's findings represent the first example of a hexagonal-shaped RNA complex. It is also the first example of transportation vehicles using RNA as building blocks.
Information from the study may someday be used to develop nanoscale devices and will also improve scientists' understanding of how cells transport large molecules through barriers such as membranes.
In the study, Guo's lab found that Bacteriophage Phi 29, a virus that infects one type of bacteria, uses six RNA to form a hexagon that acts in a manner similar to a six-cylinder car engine to drive DNA into the outer shell of the virus.
Deoxyribonucleic acid, or DNA, and ribonucleic acid, RNA, are biological molecules that allow organisms to reproduce. While DNA holds the genetic material, RNA carries out a number of functions, including the synthesis of proteins specified by DNA.
Guo says that Bacteriophage Phi 29 is typical of double-stranded DNA viruses in that its genetic material is packaged into its protein shell, or capsid, during maturation.
"All linear double-stranded DNA viruses, including herpes viruses, adenoviruses, pox viruses and the double-stranded DNA bacteriophages, package their genomic DNA into a pre-formed protein shell," he says. "What makes Phi 29 unique is that it is the first virus to be reported to use RNA as a component of the transportation machine to drive this process."
Moving large molecules, such as DNA and RNA, through barriers such as cellular membranes is a common process in biological systems. But the migration mechanisms used to package viral DNA within its outer shell are among the most complex and intricate of such processes, Guo says.
In 1987, Guo discovered this "transporting" RNA species and provided the first evidence that RNA played a role in packaging DNA in Phi 29. This new type of RNA was dubbed "pRNA" for "packaging" RNA. Subsequent reports by Guo and others have since established the presence and molecular structure of pRNA.
In his latest study, Guo shows how the structure of pRNA allows the molecules to interlock in hexagon formation, and how the hexagon functions to drive DNA through its portal in a manner similar to a bolt and a screw.
"The RNA hexagon is like a bolt that is embedded in the five-sided shell of the virus, and DNA is helical, like a screw," Guo says. "Because DNA is helical, like a screw, we envision it twisting and turning through the RNA portal, and such a five-fold/six-fold mismatch would facilitate the rotation."
Guo speculates that the force that drives this action likely comes from the contraction and relaxation of the six RNA molecules, which "fire" in a sequential manner, similar to a six-cylinder car engine.
"Some evidence for such a theory has been reported," he says.
Instead of gasoline, the DNA packaging machine uses ATP, a compound vital to producing energy processes in all living cells, to provide energy for its motor, Guo says.
"ATP provides energy to condense the lengthy DNA into the limited space inside procapsids with remarkable velocity," he says.
Though Phi 29 is the first virus reported to use RNA as a motor, other related bacterial viruses have been reported to contain pRNA, says Guo. In addition, he speculates that some RNAs may perform a similar function in animal or human cells.
Guo's studies at Purdue are funded by the National Institutes of Health and the National Science Foundation.
Web Sites: Peixuan Guo, http://biochem.purdue.edu/~bmb/faculty/quo.html
Molecular Cell, http://www.molecule.org/current.shtml
NOTE TO JOURNALISTS: For copies of the journal articles mentioned in the story, contact Susan Gaidos at Purdue News Service, (765) 494-2081.
Inter-RNA Interaction of Phage Phi 29 pRNA to Form a Hexameric Complex for Viral DNA Transportation Peixuan Guo*, Chunlin Zhang, Chaoping Chen, Chunlin Zhang, Kyle Garver and Mark Trottier
Ds-DNA viruses package their DNA into a pre-formed protein shell (procapsid) during maturation. Bacteriophage 29 requires an RNA (pRNA) to package it genomic DNA into the procapsid. We report here that the pRNA upper and lower loops are involved in RNA/RNA interactions. Mutation in only one loop results in inactive pRNAs. However, mixing of two, three and six inactive mutant pRNAs restores DNA packaging activity as long as an interlocking hexameric ring can be predicted to form by base pairing of the mutated loops in separate RNA molecules. The stoichiometry of pRNA for the packaging of one viral DNA genome is six. Homogeneous pRNA purified from a single band in denaturing gels showed six bands when rerun in native gels. These results suggest that six pRNAs form a hexameric ring by the intermolecular interaction of two RNA loops to serve as part of the DNA transportation machinery.
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