Negative and positive control - The lac repressor--operator system keeps the operon turned off in the absence of utilizable -galactosides. An overlapping regulatory system (Figure 26.21) turns the operon on only when alternative energy sources are unavailable. E. coli uses glucose in preference to most other energy substrates. When grown in a medium containing both glucose and lactose, the cells metabolize glucose exclusively until the supply is exhausted. Then the lactose operon becomes activated in preparation for continued growth using lactose. This phenomenon involves a transcriptional activation mechanism, which occurs when glucose levels are low. Control is exerted through intracellular levels of cyclic AMP.

cAMP controls in E. coli - In E. coli, cAMP levels are low when intracellular glucose levels are high. Adenylate cyclase (the enzyme that catalyzes formation of cAMP) apparently senses the intracellular level of an unidentified intermediate in glucose catabolism. Hence, the current name for the regulatory process is catabolite activation. When glucose levels drop, as shown in Figure 26.21, cAMP levels rise and cAMP interacts with a protein called cAMP receptor protein (CRP). When it binds cAMP, CRP undergoes a conformational change. The change greatly increases its affinity for certain DNA sites, including a site in the lac operon adjacent to the RNA polymerase binding site. Binding of cAMP--CRP at this site protects a DNA sequence from -68 to -55, as shown in Figure 26.19. This binding facilitates transcription of the lac operon by stimulating the binding of RNA polymerase to form a closed-promoter complex.

The cAMP--CRP complex activates several different gene systems in E. coli, all of them involved with energy generation. They include operons for utilization of other sugars, including galactose, maltose, arabinose, and sorbitol, and several amino acids. Among the operons that have been analyzed, the DNA binding site of the cAMP-activated dimer varies considerably with respect to the transcriptional start point, suggesting that regulatory mechanisms involving this protein are complex.

The CRP - DNA Complex - The structure of the CRP-cAMP-DNA complex, as revealed by x-ray crystallography, shows how the protein binds to DNA. Each CRP subunit contains a characteristic pair of helices, called a helix-turn-helix structural motif (see Figure 28.23). It is found in several DNA-binding regulatory proteins, suggesting common evolutionary origins for this family of proteins. Analysis of the DNA - protein complex shows that CRP induces DNA to bend quite sharply when it binds. This bending may facilitate the initiation of transcription by bringing DNA sequences farther upstream into direct contact with the promoter or transcriptional start site.

Transcription Regulation in Phage