longer do the job on which
E. coli’
s survival depends.
Yet
E. coli
does not die from a few degrees of extra heat. As the temperature rises, the microbe makes molecules known as heat-shock proteins. They defend
E. coli
in two ways. Some of them embrace
E. coli’
s jittery proteins and guide them back into their proper shape. Others recognize heat-snarled proteins that have been damaged beyond repair. They slice these hopeless proteins apart, leaving harmless fragments to be recycled.
Heat-shock proteins are lifesavers, but
E. coli
can’t keep a supply of them on hand for emergencies. They are among the biggest proteins in its repertoire, and to survive a blast of heat
E. coli
may need tens of thousands of them. Making heat-shock proteins in ordinary times would be like paying the local fire company to park all its trucks in your driveway just in case your house catches fire. On the other hand, when you need a fire truck, you need it fast. If
E. coli
takes too long to manufacture heat-shock proteins, it can die while it waits to be rescued.
This tricky trade-off attracted the attention of John Doyle, an engineer at the California Institute of Technology, and his colleagues. In past years, Doyle had developed a theory for designing control systems for airplanes and space shuttles. In
E. coli
he recognized a piece of natural engineering just as impressive as anything he had helped to build. He and his colleagues began to analyze its heat-shock proteins and the way
E. coli
uses them to survive.
They found that
E. coli
controls its supply of heat-shock proteins with feedback. For engineers, feedback is what happens when they allow the output of a circuit to become an input. A thermostat uses a simple form of feedback to keep the temperature of a house stable. The thermostat senses the temperature in the house and turns on the heater if it’s too cold. If the temperature gets too high, it shuts the heater down.
E. coli’
s defense against heat works a lot like a thermostat as well. The key protein in its thermostat is called sigma 32. Even when the temperature is cool,
E. coli
is constantly reading the gene for sigma 32 and making RNA copies. But at normal temperatures the RNA folds in on itself, and so
E. coli
cannot use it to make a protein. At normal temperatures the microbe is loaded with sigma 32 RNA but no actual sigma 32 protein.
Only when
E. coli
heats up can the sigma 32 RNA uncrumple. Now the ribosomes can read it and make huge amounts of sigma 32 protein. Each sigma 32 protein quickly finds some of
E. coli’
s gene-reading enzymes and leads them to the genes for heat-shock proteins.
E. coli
thus makes tens of thousands of heat-shock proteins in a matter of minutes.
Left unchecked, however, a sudden rush of sigma 32 would be too much of a good thing. The microbe would churn out heat-shock proteins far beyond its needs. In fact,
E. coli
makes just the right number of heat-shock proteins to cope with a particular temperature. It makes more proteins for higher temperatures, fewer for cooler ones. It exerts this fine control with a series of feedback loops.
E. coli’
s heat-shock proteins don’t just protect against heat. They also control the thermometer protein itself, sigma 32. Some of them grab sigma 32 and tuck it away in a pocket. Others cut it to pieces. In the first few moments of dangerous heat, heat-shock proteins are too busy helping unfolded proteins to attack sigma 32. But once they get the crisis under control, more and more heat-shock proteins become free to grab sigma 32. As the level of sigma 32 drops,
E. coli
makes fewer new heat-shock proteins.
This feedback helps keep
E. coli
from exploding with heat-shock proteins. It also controls the level of heat-shock proteins. If
E. coli
is merely warm rather than scorching, the heat-shock proteins quickly reduce the level of sigma 32. But as the temperature increases, they have to cope with more unfolded proteins, and thus they allow sigma 32 to
E. coli’
s survival depends.
Yet
E. coli
does not die from a few degrees of extra heat. As the temperature rises, the microbe makes molecules known as heat-shock proteins. They defend
E. coli
in two ways. Some of them embrace
E. coli’
s jittery proteins and guide them back into their proper shape. Others recognize heat-snarled proteins that have been damaged beyond repair. They slice these hopeless proteins apart, leaving harmless fragments to be recycled.
Heat-shock proteins are lifesavers, but
E. coli
can’t keep a supply of them on hand for emergencies. They are among the biggest proteins in its repertoire, and to survive a blast of heat
E. coli
may need tens of thousands of them. Making heat-shock proteins in ordinary times would be like paying the local fire company to park all its trucks in your driveway just in case your house catches fire. On the other hand, when you need a fire truck, you need it fast. If
E. coli
takes too long to manufacture heat-shock proteins, it can die while it waits to be rescued.
This tricky trade-off attracted the attention of John Doyle, an engineer at the California Institute of Technology, and his colleagues. In past years, Doyle had developed a theory for designing control systems for airplanes and space shuttles. In
E. coli
he recognized a piece of natural engineering just as impressive as anything he had helped to build. He and his colleagues began to analyze its heat-shock proteins and the way
E. coli
uses them to survive.
They found that
E. coli
controls its supply of heat-shock proteins with feedback. For engineers, feedback is what happens when they allow the output of a circuit to become an input. A thermostat uses a simple form of feedback to keep the temperature of a house stable. The thermostat senses the temperature in the house and turns on the heater if it’s too cold. If the temperature gets too high, it shuts the heater down.
E. coli’
s defense against heat works a lot like a thermostat as well. The key protein in its thermostat is called sigma 32. Even when the temperature is cool,
E. coli
is constantly reading the gene for sigma 32 and making RNA copies. But at normal temperatures the RNA folds in on itself, and so
E. coli
cannot use it to make a protein. At normal temperatures the microbe is loaded with sigma 32 RNA but no actual sigma 32 protein.
Only when
E. coli
heats up can the sigma 32 RNA uncrumple. Now the ribosomes can read it and make huge amounts of sigma 32 protein. Each sigma 32 protein quickly finds some of
E. coli’
s gene-reading enzymes and leads them to the genes for heat-shock proteins.
E. coli
thus makes tens of thousands of heat-shock proteins in a matter of minutes.
Left unchecked, however, a sudden rush of sigma 32 would be too much of a good thing. The microbe would churn out heat-shock proteins far beyond its needs. In fact,
E. coli
makes just the right number of heat-shock proteins to cope with a particular temperature. It makes more proteins for higher temperatures, fewer for cooler ones. It exerts this fine control with a series of feedback loops.
E. coli’
s heat-shock proteins don’t just protect against heat. They also control the thermometer protein itself, sigma 32. Some of them grab sigma 32 and tuck it away in a pocket. Others cut it to pieces. In the first few moments of dangerous heat, heat-shock proteins are too busy helping unfolded proteins to attack sigma 32. But once they get the crisis under control, more and more heat-shock proteins become free to grab sigma 32. As the level of sigma 32 drops,
E. coli
makes fewer new heat-shock proteins.
This feedback helps keep
E. coli
from exploding with heat-shock proteins. It also controls the level of heat-shock proteins. If
E. coli
is merely warm rather than scorching, the heat-shock proteins quickly reduce the level of sigma 32. But as the temperature increases, they have to cope with more unfolded proteins, and thus they allow sigma 32 to
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