“ Escherichia coli is a Gram-negative, non-sporulating, facultative anaerobic bacterium that is commonly found in the lower gastrointestinal tract of warm-blooded animals.”
Escherichia coli This is the mapped genome of the E. coli bacteria. Note the lac operon.
“ An operon is a functioning unit of key nucleotide sequences including an operator , a common promoter , and one or more structural genes , which are controlled as a unit to produce messenger RNA (mRNA), in the process of protein transcription.”
Promoter P/O Overlap Operator Coding Region T-Terminator
“ The lac operon is an operon required for the transport and metabolism of lactose … It consists of three adjacent structural genes, a promoter, a terminator, and an operator. The lac operon is regulated by several factors including the availability of glucose and of lactose.”
The lac operon is environmentally regulated . This means that external stimuli will affect whether or not it is expressed.
In this case, the presence of lactose changes the conformation of LacI to bind to the P/O overlap region, blocking RNAP from creating mRNA for the lac operon.
NB: LacI is not synthesized from the lac operon, it is on a separate part of the genome!
lac Operon The RNAP binds at the promoter region, then travels along the operator and the coding regions for lacZ, lacY and lacA. However, note that the P/O overlap has a binding site for a repressor . A repressor will “get in the way” of RNAP.
lac Operon (with Lactose) RNAP LacZ LacY LacA KEY IDEA: THERE IS LACTOSE! Lactose can be digested!
lac Operon (without Lactose) RNAP LacI KEY IDEA: THERE ISN'T LACTOSE! No lactose to digest!
Please refer to the custom notes for a more detailed explanation of this mechanism!