What do transposons and other transposable elements have to do with the genetics of prokaryotes—or with hospitals? Transposable elements have contributed to the evolution of plasmids. R factors probably originally gained their genes for antibiotic resistance through the activity of transposable elements. One piece of evidence for this conclusion is that each resistance gene in an R factor is part of a transposon.

In summary, rapid asexual reproduction can produce enormous clones of prokaryotes. However, these genetically identical cells are all equally vulnerable to some change in the environment. Recombination by means of conjugation, transformation, and transduction, or the acquisition of new genes by means of plasmids and transposable elements, all introduce genetic diversity into bacterial populations, and this diversity allows at least some cells to survive under changing conditions. Prokaryotes can also respond to changes in their environment by regulating the expression of their genes.

Regulation of Gene Expression in Prokaryotes

Prokaryotes can conserve energy and resources by making proteins only when they are needed. The protein content of a bacterium can change rapidly when conditions warrant. There are several ways in which a prokaryotic cell could shut off the supply of an unneeded protein:

► Block the transcription of mRNA for that protein

► Hydrolyze the mRNA after it is made and prior to translation

► Prevent translation of the mRNA at the ribosome

► Hydrolyze the protein after it is made

► Inhibit the function of the protein

These methods would all have to be selective, affecting some genes and proteins and not others. In addition, they would all have to respond to some biochemical signal. Clearly, the earlier the cell intervenes in the process, the less energy it has to expend. Selective inhibition of transcription is far more efficient than transcribing the gene, translating the message, and then degrading or inhibiting the protein. While examples of all five mechanisms for regulating protein levels are found in nature, prokaryotes generally use the most efficient one, transcriptional regulation.

Regulation of transcription conserves energy

As a normal inhabitant of the human intestine, E. coli must be able to adjust to sudden changes in its chemical environment. Its host may present it with one foodstuff one hour and another the next. This variation presents the bacterium with a metabolic challenge. Glucose is its preferred energy source, and is the easiest sugar to metabolize, but not all of its host's foods contain an abundant supply of glucose. For example, the bacterium may suddenly be deluged with milk, whose predominant sugar is lactose. Lactose is a P-galactoside—a disaccharide containing galactose P-linked to glucose (see Chapter 3). To be taken up and metabolized by E. coli, lactose is acted on by three proteins:

► b-galactoside permease is a carrier protein in the bacterial plasma membrane that moves the sugar into the cells.

► b-galactosidase is an enzyme that catalyzes the hydrolysis of lactose to glucose and galactose.

► A third protein, the enzyme b-galactoside transacetylase, is also required for lactose metabolism, although its role in the process is not yet clear.

When E. coli is grown on a medium that does not contain lactose or other P-galactosides, the levels of these three proteins are extremely low—the cell does not waste energy and materials making the unneeded enzymes. If, however, the environment changes such that lactose is the predominant sugar available and very little glucose is present, the bacterium promptly begins making all three enzymes, and they increase rapidly in abundance. For example, there are only two molecules of P-galactosidase present in an E. coli cell when glucose is present in the medium. But when glucose is absent, lactose can induce the synthesis of 3,000 molecules of P-galactosidase per cell!

If lactose is removed from E. coli's environment, synthesis of the three enzymes that process it stops almost immediately. The enzyme molecules that have already formed do not disappear; they are merely diluted during subsequent cell divisions until their concentration falls to the original low level within each bacterium.

Compounds that stimulate the synthesis of an enzyme (such as lactose in our example) are called inducers (Figure 13.13). The enzymes that are produced are called inducible a

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Essentials of Human Physiology

Essentials of Human Physiology

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