This account of the control of internal states might seem incomplete. We have made it appear that cells just go on expressing genes, sometimes faster, sometimes slower, but making more and more proteins all the time. This is true; but cells do not choke or burst as a result of overproduction, because neither proteins nor messenger RNA molecules last forever. Indeed, some messengers, and some proteins, have very short life spans - just a minute or two. Proteins can be marked for rapid removal by attachment of a "tag". This tag ensures they are quickly destroyed or otherwise rendered inactive, a necessary precaution for proteins that are needed for some immediate purpose but might be harmful if they lasted longer.
Most eukaryotic proteins and messenger RNAs survive for hours or days rather than minutes; but like all machines, proteins wear out. Worn-out proteins are digested by proteasomes (see chapter 3). The component parts of the digested proteins (amino acids) are recycled. Messenger RNA molecules meet a similar fate. Even the longest-lived proteins and messengers will be removed sooner or later. Thus, proteins are continually being destroyed as well as made, so the cell does not become overloaded.
The removal of proteins alters the internal state, just as protein manufacture does. A cell's internal state depends on its protein composition, and in principle this can be as well be changed by removal as by addition of proteins. However, the control of protein breakdown has not been nearly so intensively studied as the control of gene expression. Overall, it is probably less important for altering a cell's internal state because, so far as we know at present, it is subject to less elaborate controls. It is dangerous to be dogmatic about this; we might have to eat our words in ten years' time. But in chapter 8 we shall offer some justification for our claim that the control of gene expression has a more important role than the control of protein breakdown in regulating and manipulating the cell's internal state.
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