Question

How is the control gene of the Arabinose Operon different from the Lactose Operon?

Answer 1

The operator-repressor-inducer interactions provide an intuitively satisfying model for an on/off switch in the regulation of gene expression. Years of research, however, have shown that operon regulation is rarely so simple. Even a bacterium has a highly complex environment too complex for it to have sets of genes sensitive to only one signal. Another major environmental factor affecting the expression of the lac genes is the presence or absence of glucose. Glucose is the preferred cellular energy source because of its central place in cellular metabolism. Hence, expressing the genes required to metabolize sugars such as lactose, galactose, and arabinose would be wasteful if glucose were abundant. A more complex regulatory scheme is found in the arabinose (c~ra) operon of E. coli. This system introduces several additional regulatory mechanisms. First, it is possible for one regulatory protein to exact both positive and negative control. In this case the regulatory protein is the AraC protein, and binding of a signal molecule alters its conformation from a repressor form that binds one DNA regulatory sequence to an activator that binds a different DNA sequence. Second, the AraC protein regulates its own synthesis by repressing transcription of its gene. This phenomenon is called autoregulation. Finally, the effects of some regulatory DNA sequences can be exerted from a distance; that is, these sequences are not always contiguous with promoters. Distant DNA sequences can be brought into proximity by DNA looping, mediated by specific protein-protein and protein-DNA interactions. This last feature makes the ara system an important paradigm for eukaryotic gene expression, in which regulation involving relatively distant sites in the DNA is quite common.