A Switch is Born

Carl Zimmer in his blog, The Loom:

All living things have genes. Enzymes read those genes and produce a copy of their code, which a cell can then use to build a protein. But in order to read a gene, the enzymes must first lock onto a distinctive segment of DNA near the gene, known as a promoter. Promoters act like switches, which a cell can use to turn genes on and off. Different genes carry different promoters, so that they can be switched on under different conditions.

EscherichiaScientists have studied the promoters of the bacteria Escherichia coli more closely than those of any other species, and they’ve identified some of its switching patterns. When Escherichia coli is growing quickly, it produces a lot of gene-reading enzymes factors called sigma 70. Sigma 70 can switch on several hundred genes that allow the microbe to feed and build up its biomass and reproduce. If Escherichia coli begins to starve, it slips into a sort of suspended animation, and produces a different enzyme factor called sigma S. Sigma S recognizes a different set of genes that begin to make the proteins necessary for shutting the microbe’s operations down.

Here we have a wonderfully precise system for controlling genes. Now imagine that Escherichia coli acquires a gene with no promoter at all–just a random sequence of DNA next to the gene, 41 nucleotides long. Imagine that this DNA starts going through cycles of mutation and natural selection. Would it be possible for a random sequence to change into one Sigma 70 could grab? Could it go from nothing to a promoter?

The answer is yes. How long would it take? According to some recent experiments, two days. Two.

More here.

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