MoBio The lac Operon Promoter Chapter 4

The lac operon of E. coli was used by Francois Jacob, Andre Lwoff and Jacques Monod to investigate the transcription mechanism in 1960s, providing the first molecular details. Its promoter sequence is shown in the following figure.


Figure 4-C-2. The lac operon promoter ( which regulates the transcription of lacZ, not lacI).

The DNA's two strands are complementary to each other. Only one strand's sequence is sufficient to represent both strands' information. By convention, the sequence of the 5' to 3' strand (from left to right) is used. In the above figure, the promoter sequence of the lac operon is TATGTT at the -10 region, and TTTACA at the -35 region. From Table 4-C-1, the consensus sequence of σ70 which regulates the lac operon is TATAAT at the -10 region, and TTGACA at the -35 region. They differ at three positions (two at the -10 region and one at the -35 region). Experimentally, it has been found that binding of the RNA polymerase to the lac promoter is relatively weak. Transcription of the lac operon often requires an activator such as catabolite activator protein (CAP).