Question

Describe the steps of how eukaryotes control gene expression using transcriptional and post transcriptional .

Explain in details steps clearly and if possible compare it to why they use this and not OPERONS

Answer 1

Eukaryotes are a more complex and diverse range of organisms. When compared with a prokaryote, they have better nuclear organisation and more developed cellular organelles. As a result the functions of various factors and genes in this type of cells are diverse and require a more organised and specific approach to manufacture proteins and operate the cellular machinery.

Regulation of gene expression refers to the control in the amount and timing of the product expressed by the gene. This way a cell is able to produce anly those proteins and factors from its catalogue that it requires in the immediate future in necessary amounts, instead of producing proteins with no immediate use.

The types of genes depending on how they are regulated can be classified as follows -

• Constitutive Genes - These genes are transcribed continously.
• Facultative Genes - They are only transcribed when needed.
• Inducible Genes - Its expression depends on environmental triggers or only during specific phases in a cells lifecycle.

Regulation of gene expression during the transcriptional stages -

• Eukaryotes have three RNA polymerases which are regulated by independent mechanisms. Generally polymerases can be regulated through the following methods-

1. Controlling the polymerases' access to the gene - Done through histone remodelling, enhancers & repressors.
2. Productive elongation of the RNA transcript - When polymerase binds to the promoter, it requires factors to escape the promotor complex so that elongation can occur.
3. Termination of the polymerase - Many factors control when and how termination occurs in the polymerase.

• Regulation at the chromatin state -The genetic information is packed in the chromatin material. Here the DNA is bound and packed efficiently to the histone proteins.In addition the DNA sequuence is also silenced by processes such as methylation and ubiquitinisation. For the polymerases to bind and begin transcription, the DNA has to be unmethylated. This way the sequestered genes would be available for mRNA production. Histone methyltransferases are used to demethyl the DNA sequence and regulate expression at this stage.
• Regulation through Enhancer - They are non coding DNA containing multiple repressor and activator binding sites. They can bind to promoters through the core DNA binding motif promoter specificity. They can affect more than one gene without linkage restriction
• Regulation of the Pre-initiation Complex: Transcription factors TFIID & TFIIA bind to the core promoter followed by TFIIB creating a stable complex to which other transcription factors are able to bind to. RNA polymerase II also binds and the complex undergoes phosphorylation of the C terminal domain. Many transcription facters employed here have kinase activity and hence this phosphorylation promotes transcription and act as a site to recruit the components of the mRNA proessing machinery. If the phosphorylation does not occur, the polymerase II binds and blocks the promoter and transcription is stalled.

Regulation of gene expression during the post transcriptional stages -

• After translation, the RNA transcrips are bound by RNA binding proteins that determine their fate. Whether the transcripts are to be translated, degrated, sequestered in P bodies or exported to the nucleus for modification.
• Based on the sequence of the transcripts, they undergo various modifications to regulate end product formation.

1. Capping - The five prime end of the mRNA is changed to a three prime end which protects the transcript from five prime endonuclease which destroy new mRNA. It also facilitates ribosomal binding.
2. Splicing - Removes the intronic sequences between the mRNA using a spliceosome complex. The exon joins and makes a functional transcript.
3. Addition of poly A tail - Poly adenylation adds a 3' tail of adenine residues that increases the half life of the transcript by delaying its destruction by the endonucleases. Also, it allows the binding of the Poly A binding protein which is the site for the beginning of translation.

• MicroRNA - Abundance of microRNA determines their ability ot act as a switch to turn genes on/off. They can have an effect to reduce or increase protein expression of their targeted genes by four times. microRNA are able to repress hundreds of genes either by degrading the mRNA by complementary binding or through translational silencing of mRNA. Both are achieved through the RNA induced silencing complex.

Eukaryotic cells prefer controlling gene expression at various stages of the protein manufacturing process instead of using an operon as it gives the cellular machinery more control in manufacturing the proteins. Some protein may have a common precursor but may generate different functions after post translational modification. This process can be easily optimised in a eukaryotic cell but requires two separate operons in a prokaryotic cell. The manufacturing process is more efficient and the quantities generated can be regulated by the cell in real time unlike in the case of operons.