Genomes are made up of DNA that contains the recipes for proteins, but one crucial part of a cell's nucleus may have a unique kind of genome of its own, one spelled out in RNA, which is best known as a go-between in the production of proteins from genes. That means the oddball entity could be a molecular relic from the RNA world, which predated the evolution of DNA, lending credence to the idea that life depended first on RNA.
RNA is present in the centrosome, a glob of proteins essential to cell division, report cell biologist Mark Alliegro of Louisiana State University Health Sciences Center in New Orleans and colleagues. Centrosomes are key to cell division and move to opposite edges of a cell before it splits. They anchor strands of protein that attach to replicated chromosomes and help pull the paired chromosomes apart, so that each new cell gets just one copy of each chromosome. In anticipation of this task, a centrosome itself replicates. Researchers have argued for decades whether these anchors have their own "genome," and if so, whether its nucleic acid sequence is DNA or RNA.
Alliegro, his wife Mary Anne, also at LSU, and cell biologist Robert Palazzo from Rensselear Polytechnic Institute in Troy, New York, have begun to resolve this debate. Palazzo painstakingly picked out centrosomes from large and easily manipulated surf clam eggs and detected nucleic acids in the concentrated extract. The Alliegros analyzed this RNA, finding five sequences that looked nothing like any known RNA and that they couldn’t find any place else in the egg. One piece codes for an enzyme involved in replicating RNA called reverse transcriptase, the researchers report online this week in the Proceedings of the National Academy of Sciences. If produced, this enzyme might enable the centrosome to copy its RNA as it forms a second centrosome.
Other organelles within a cell, such as mitochondria and chloroplasts, have their own genomes spelled out in DNA. The centrosome seems to have just RNA, Palazzo points out. However, even with a reverse transcriptase gene in the genome, the RNA may not replicate, but instead could simply help organize the centrosome or hold it together, he cautions.
The mere existence of this RNA, let alone its function, will likely reignite controversy about centrosomes, says Joel Rosenbaum, a cell biologist at Yale University. And the group will have to rule out viruses as the source of the RNA. But the work is still important in resolving some of the mystery about this cell component, Rosenbaum notes. "They have gone a step further than others have in the past."