Dernburg and Phillips applied "reverse genetics" to the neighboring genes, obtaining worms from the Japanese National Bioresource in which each of the three new genes had been specifically deleted. By observing meiosis (and its failures) in these knock-out worms, it was clear that one of the new genes was responsible for successful pairing of the nonsex chromosomes II and III, one was responsible for pairing of chromosome V, and the third for chromosomes I and IV.
The researchers were able to visually detect the zinc-finger proteins during meiosis and show that each protein binds specifically to the chromosomes they help to pair. Further tracking showed that during meiosis the HIM-8 and ZIM proteins on each chromosome latch onto the cell's nuclear envelope. This activity is reminiscent of the way chromosomes attach to the nuclear envelope during meiosis in other organisms.
"In most eukaryotic species -- plants, mammals, fungi, and so on - the telomeres on the ends of the chromosomes anchor to the nuclear membrane in a transient structure called a 'meiotic bouquet,'" Dernburg says. "In C. elegans it's the Pairing Centers, not telomeres, that attach to the nuclear envelope. They do so seemingly randomly, not in a single bunch; nevertheless, the association with the nuclear envelope seems to serve a similar function, which is to stabilize chromosome interactions during pairing."
Exactly how, Dernburg says, is still unknown. "Zinc-finger proteins evolve rapidly because they are modular, so that combinations of elements allow them to move quickly to new binding sites on DNA, making the Pairing Centers of different chromosomes unique," she says. "Telomeres, on the other hand, are all very much alike, no matter which chromosome they're on. It could be that C. elegans, which is hermaphroditic and grows to maturity in only three
Contact: Paul Preuss
DOE/Lawrence Berkeley National Laboratory