Microcosm_ E. Coli and the New Science of Life - Carl Zimmer [26]
Once it becomes eager, E. coli will resist changing back. If the concentration of lactose drops, the microbe will still pump in lactose at a high rate, thanks to all the permease channels it has built. It can supply itself with enough lactose to keep the repressors away from the operon so that it can continue making beta-galactosidase and permease. Only if the lactose concentrations drop below a critical level do the repressors suddenly get the upper hand. Then they shut the operon down, and the microbe turns off.
This sticky switch helps to make sense of Novick and Weiner’s strange experiments. Two genetically identical E. coli can respond differently to the same level of lactose because they have different histories. The reluctant one resists being switched on while the eager one resists being switched off. And both kinds can pass on their state to their offspring. They don’t bequeath different genes to their descendants. Some give their offspring a lot of permeases on their membranes and a lot of lactose molecules floating through their interiors. Others give their offspring neither.
Combine this peculiar switch with E. coli’s unpredictable bursts and you have a recipe for individuality. If a colony of E. coli encounters some lactose, some of the bacteria will respond with a huge burst of proteins from their lac operon. They will push themselves over the threshold from reluctant to eager, and they will stay that way even if the lactose drops. Other E. coli will respond to the lactose with no proteins at all. They will remain reluctant. These clones take on different personalities thanks to chance alone.
E. coli also gets some of its personality from an extra layer of heredity. Some of its DNA is covered with caps made of hydrogen and carbon atoms. These caps, known as methyl groups, change the response of E. coli’s genes to incoming signals. They can, in effect, shut a gene down for a microbe’s entire life without harming the gene itself. When E. coli divides in two, it bequeaths its pattern of methyl groups to its offspring. But under certain conditions, E. coli will pull methyl groups off its DNA and put new groups on—for reasons scientists don’t yet understand.
Some of the factors that spin the wheel for E. coli spin it for us as well. To smell, for example, we depend on hundreds of different receptors on the nerve endings in our noses. Each neuron makes only one type of receptor. Which receptor it makes seems to be a matter of chance, determined by the unpredictable bursts of proteins within each neuron. Our DNA carries methyl groups as well, and over our lifetime their pattern can change. Pure chance may be responsible for some changes; nutrients and toxins may trigger others. Identical twins may have identical genes, but their methyl groups are distinctive by the time they are born and become increasingly different as the years pass. As the patterns change, people become more or less vulnerable to cancer or other diseases. This experience may be the reason why identical twins often die many years apart. They are not identical after all.
These different patterns are also one reason why clones of humans and animals can never be perfect replicas. In 2002, scientists in Texas reported that they had used DNA from a calico cat named Rainbow to create the first cloned kitten, which they named Cc. But Cc is not a carbon copy of Rainbow. Rainbow is white with splotches of brown, tan, and gold. Cc has gray stripes. Rainbow is shy. Cc is outgoing. Rainbow is heavy, and Cc is sleek. New methylation patterns probably account for some of those differences. Clones may also get hit by a unique series of protein bursts. The very molecules that make them up turn them into individuals in their own right.
At the very least, E. coli’s individuality should be a warning to those who would put human nature down to any sort of simple genetic determinism.