ocess could be at work in HIV. No one had ever shown that this type of "noise" in gene expression could influence phenotype in infected mammalian cells, Weinberger said.
"I thought it was a cool idea," he added, but "at the time, there wasn't a lot of data to support it. It was pretty far out there."
Weinberger, Arkin, and their colleagues created a model HIV-1 vector--a virus that could enter human cells, carrying with it a key component of HIV's replication machinery: a gene called Tat. Tat facilitates transcription of HIV's entire genome, including itself, which creates a positive-feedback loop: If a little bit of Tat is around, then the HIV genome is transcribed efficiently, which makes more Tat, and so on. If a cell has no Tat, then the HIV genome may remain untranscribed, and, unable to replicate, so the virus heads for latency.
When the scientists infected cultured human cells with their viral vector, they found that cells that initially expressed a low level of virus were very unstable: After a few days, all cells expressed either a high level of virus or none at all.
When Weinberger took one infected human cell with low levels of the virus and allowed it to proliferate into many genetically identical copies of itself, he found that these progeny did not all show the same behavior: some turned their viral expression on high and others turned it off.
This dichotomy in cell fate from genetically identical cells is consistent with the idea that random fluctuations in gene expression control what happens to the cells, the researchers said. To see if this really was the best explanation, they conducted a large array of experimental controls to discount other hypotheses and then created computer programs to model what would happen to the virus under different cellular conditions.
Of the 16 models they tested, only one produced results that matched those seen experimentally in the infected cells. In this model, after
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Source:Howard Hughes Medical Institute
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