Question

What are the specific players in the lac operon in E. coli and S. cerevisiae (trans-acting factors and cis-elements)?

Answer 1

• The lac operon is an operon required for the transport and metabolism of lactose in E.coli.
• Three of the enzymes for lactose metabolism are grouped in the lac operon: lacZ, lacY, and lacA.
• LacZ codes for an enzyme called β-galactosidase, which digests lactose into its two constituent sugars: glucose and galactose. lacY is a permease that promotes the transfer of lactose into the cell. LacA is a trans-acetylase that transfers an acetyl group from acetyl-CoA to galactosides, glucosides and lactosides.
• In addition to the these genes, the lac operon contains short DNA sequences that do not code for any proteins, but are instead binding sites for proteins involved in transcriptional regulation of the operon.These sequences are called P (promoter), O (operator), and CBS (CAP-binding site). Sequence elements such as these are collectively called cis-elements because they must be located on the same piece of DNA as the genes they regulate.
• On the contrary, the proteins that bind to these cis-elements are called trans-regulators because (as diffusible molecules) they do not necessarily have to be coded by the same piece of DNA as the genes they regulate.
• One of the most important trans-regulators of the lac operon is coded by lacI. Four molecules of lacI proteins assemble together to form a tetramer called a repressor. This repressor binds to two operator sequences adjacent to the promoter of the lac operon. Binding of the repressor prevents RNA polymerase from binding to the promoter thus preventing transcription of the structural genes.

The basic structure is as follows :-

The functioning is illustrated as follows :-