Within its tiny cell, the bacterium E. coli contains all the genetic information it needs to metabolize, grow, and reproduce. It can synthesize every organic molecule it needs from glucose and a number of inorganic ions.
Many of the genes in E. coli are expressed constitutively; that is, they are always turned "on". Others, however, are active only when their products are needed by the cell, so their expression must be regulated.
Two examples:- If the amino acid tryptophan (Trp) is added to the culture, the bacteria soon stop producing the five enzymes previously needed to synthesize Trp from intermediates produced during the respiration of glucose. In this case, the presence of the products of enzyme action represses enzyme synthesis.
- Conversely, adding a new substrate to the culture medium may induce the formation of new enzymes capable of metabolizing that substrate. If we take a culture of E. coli that is feeding on glucose and transfer some of the cells to a medium contain lactose instead, a revealing sequence of events takes place.
- At first the cells are quiescent: they do not metabolize the lactose, their other metabolic activities decline, and cell division ceases.
- Soon, however, the culture begins growing rapidly again with the lactose being rapidly consumed. What has happened? During the quiescent interval, the cells began to produce three enzymes.
The three enzymes are:- a permease that transports lactose across the plasma membrane from the culture medium into the interior of the cell
- beta-galactosidase which converts lactose into the intermediate allolactose and then hydrolyzes this into glucose and galactose. Once in the presence of lactose, the quantity of beta-galactosidase in the cells rises from a tiny amount to almost 2% of the weight of the cell.
- a transacetylase whose function is still uncertain.
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