Question

27. Be able to describe regulation of gene expression in prokaryotes (transcriptional, like the Lac Operon) and in eukaryotes (transcriptional and epigenetic regulation).

Answer 1

Gene regulatory mechanisms in eukaryotes:

1. Epigenetic modifications: These modifications alter the recruitment of RNA polymerase to the gene locus by changing the compactness of the chromatin. Ex: Histone modifications and DNA methylation.

2. Transcriptional gene regulation: Activators and repressors regulate the recruitment of RNA polymerase to the target gene loci.

3. Post-transcriptional gene regulation: Regulation of mRNA stability, splicing and miRNAs/siRNAs.

4. Translational gene regulation: Regulation of ribosome recruitment to the mRNA

5. Post-translation regulation: Regulation of protein folding, localization, covalent modification, protein-protein interactions, and protein stability.

Gene regulation mechanisms in prokaryotes:

Regulation of gene expression in prokaryotes is predominantly at the transcriptional level. Prokaryotes contain multiple genes that participate in a common pathway in the form of an operon.

Lac operon contains three structural genes under the control of a common regulatory circuit. It is an example of inducible negative regulation. It is a catabolic operon. The structural genes code for enzymes required for the catabolism of lactose.

Lac Z = Beta-galactosidase = required for the hydrolysis of lactose

Lac Y = Permease = required for the transport of lactose

Lac A = Transacetylase

When lactose is absent, Lac I binds to the operator and prevents the recruitment of RNA polymerase to the promoter.

When lactose is available, it binds to Lac I and prevents its binding to the operator. Now, the promoter is free to be bound by the RNA polymerase and transcription of the structural genes takes place.

trp operon is a biosynthetic operon. It is an example of repressible negative regulation. It contains five structural genes which are required for the biosynthesis of tryptophan.

When Trp is present, an anti-terminator sequence is formed which prevents RNA polymerase from transcribing the operon.

When Trp is absent, RNA polymerase transcribes the operon.