Question

1) Explain how eukaryotic gene expression is controlled from a distance

2) Describe the experiments that showed all differentiated cells contain all the genetic instructions to direct the formation of a complete organism.

3) Explain the structure of tryphtophan operon and how this operon is regulated at the low and high concentration of tryphtophan.

Answer 1

1. Eukaryotes have three different classes of RNA polymerases and can be distinguished on the basis of their relative locations:

• At the site where transcription initiates is the core promoter (the RNA polymerase II binding region) &
• promoter-proximal cis-acting sequences that bind to proteins which in turn assist the binding of RNA polymerase II to its promoter.

Additional cis-acting sequence elements act at considerate distance which are termed as enhancers and silencers.

The two classes of cis-acting elements which exerts their effects from a considerable distance from the promoter are:

1. Enhancers - can greatly increase transcription rates from promoters on the same DNA molecule; hence they activate or +vely regulate transcription. Whereas, silencers have exact opposite effect.
2. Silencers - are bound by repressors, thereby inhibiting activators and reducing the transcription.

Enhancers and silencers are like promoter-proximal regions as they are organised as a series of cis-acting sequences which are bound by trans-acting regulatory proteins. However, they are different from promoter-proximal elements by being able to act at a distance of 50 kb or greater, and able to operate either upstream or downstream from the promoter they control.

Enhancer and silencer elements regulate transcription from many thousands of base pairs away by DNA looping. A DNA loop brings activator proteins bound to distant enhancer elements into proximity to protein complexes associated with promoter-proximal cis-acting sequences. Enhancers can act at long distance and can activate many different promoters. A gene enhancer at one breakpoint can regulate the transcription near other breakpoint.

TATA box Transcribed region TAFS ID ID Inhibitor IID Inhibitor IBD IIB LAB Poll Pol II 2 IIF IID Poll Pol II TIBIF II E IIH ID Pol II IBF II J NA TIE IH

Ref: H. Lodish, D. Baltimore, A. Berk, S. L. Zipursky, P. Matsudaira, and J. Darnell, Molecular Cell Biology, 3d ed. Copyright © 1995 by Scientific American Books.

• RNA polymerase II initiation complex assembly starts with binding of TFIID to TATA box.
• TFIID is composed of one TATA box-binding subunit - TBP (dark blue) & greater than 8 other subunits - TAFs (light blue).
• Binding of inhibitors to the TFIID-promoter complex block the binding of other transcription factors.
• TFIIA binding to the TFIID-promoter complex (DA complex) prevents binding of inhibitor.
• Binding of TFIIB to the DA complex is followed by preformed complex binding between TFIIF & RNA polymerase II.
• For initiation of transcription TFIIE, TFIIH, and TFIIJ finally add to the complex orderly.