"When you remove both proteins, the plants die as embryos; but if they lack just one of the proteins, they still survive, which is evidence that the two alternative forms of the protein are redundant for survival," he said. "But despite this, plants missing either 9a or 9b have different physical characteristics, such as leaf shape, suggesting that Pol II built using 9a does not function exactly the same as Pol II assembled using 9b."
Another unique feature found between the two protein subunits involves the functionality of Pol V and its ability to conduct RNA-directed DNA methylation: The Pol V polymerase built using 9b facilitates methylation, while the 9a-built Pol V does not.
"This is the first evidence showing that different functional subtypes of nuclear RNA polymerases are generated using alternative subunits, and there are multiple subunits for which more than one variant is produced," Pikaard said. "The results also show for the first time that the ninth subunit has a role in RNA-directed DNA methylation."
With new evidence from other research that RNA-directed DNA methylation and transposon silencing also takes place in the sperm-forming cell lineage in mammals, and not just in plants, Pol II transcription is implicated in methylation in both plants and animals.
"Alterations in DNA methylation and gene silencing are involved in multiple genetic disorders and diseases, including cancer," Pikaard said. "Our studies of RNA Pol IV and Pol V may tell us important things about their cousin, Pol II, that may not be possible to know otherwise, including how RNA synthesis can help specify sites of DNA methylation."
|Contact: Steve Chaplin|