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A new study finds that amounts of protein (green) and mRNA (red) vary from cell to cell.Y. Taniguchi, P. Choi, G. Li and H. Chen

Using new technology to peer inside single cells and count individual molecules, researchers have found that there’s a lot of variability among these biochemical factories even when they’re working from the same set of plans.

In the past, scientists have studied what goes on in cells by looking at them en masse and have assumed that all of them are pretty much the same. But a study in the July 30 Science finds substantial differences from one E. coli bacterium to the next in the relative abundance of cellular proteins and the RNA molecules that encode them.

The “central dogma” in molecular biology is that DNA is copied into messenger RNA, which is the blueprint for proteins. Scientists have long assumed that the amount of mRNA in a cell is proportional to the amount of its associated protein. But now researchers are checking to see if that’s the case for individual cells.

“We know the central dogma, but we want to understand it at the quantitative level,” says biophysical chemist Sunney Xie of Harvard University, who led the study.

Xie and his team looked inside cells of the gut microbe Escherichia coli and did the first broad analysis of the exact protein and mRNA numbers in single cells. A special microscope technique allowed the researchers to simultaneously count individual mRNAs and proteins associated with 1,018 genes, about a quarter of the microbe’s genome. They found that the number of proteins in a single cell wasn’t at all related to the number of associated mRNAs.

“That sounds absurd on first examination,” says cell biologist Sanjay Tyagi from the University of Medicine and Dentistry of New Jersey in Newark. “You expect that there should be direct correspondence between the number of molecules of RNA the cell has and the number of proteins it has.”

But when you consider how mRNA and proteins work, he says, the result makes sense.

Protein and mRNA molecules exist on two different time scales, Xie explains. Messenger RNA molecules are short lived; they degrade just a minute or two after forming. That means their number varies depending exactly when the count is taken.

Proteins, on the other hand, stick around for hours after they’re made. They often last until the cell divides, so their numbers are more constant over time.

The results provide a “cautionary note” to researchers when they measure mRNA levels in single cells, Xie says: They need to take into account that the mRNA level in a cell does not reflect the level of its associated protein. Xie and his team next will study how this “noise” might contribute to antibiotic resistance in bacteria.

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