Chapter 16 – Control of Gene Expression in Prokaryotes

Genes Structural genes Regulatory genes Regulatory elements Code for proteins involved with general processes (metabolism, catabolism) or structural components of cell Regulatory genes Code for RNA/proteins that affect transcription/translation of other sequences Usually by binding to DNA Regulatory elements Sequences of DNA that are not transcribed Site of binding to regulatory proteins

Levels of Gene Regulation Alteration of gene structure More common in eukaryotes – hetero – vs euchromatin Transcriptional control Whether RNA is created or not mRNA processing Post-transcriptional modifications in eukaryotes Stability of RNA Degradation of mRNA Translational control Whether or not translation occurs

DNA binding proteins Domain Motifs Helix-turn-helix Region of regulatory protein that binds to DNA Approx 60-90 a.a. Motifs Simple structure of regulatory proteins Helix-turn-helix Common in prokaryotes Binds to major groove of DNA

DNA binding proteins cont Zinc fingers Common in eukaryotes Binds to major groove of DNA Leucine zipper Binds to two adjacent major grooves of DNA

Prokaryotic Operon Structure In prokaryotes, genes with similar functions are clustered together and are under the control of the same promotor Transcribed as a single mRNA Operon is promotor, operator, and structural genes Promotor – site for RNA polymerase binding Operator – “on/off” switch; determines if transcription will occur or not Regulator Not part of an operon Codes for a regulatory protein that binds to the operator

Gene Control Classified by regulatory protein function Negative control Repressor function – inhibits transcription Positive control Activator function – stimulates transcription Classified by “resting” state of operon Inducible Transcription is usually “off”; needs to be activated Ex: to make enzymes that are necessary only when substrate is present Repressible Transcription is usually “on”; needs to be silenced Ex: gene products are always needed for cell functioning, unless already in high concentration

Lac operon in E. coli Three genes for lactose metabolism LacZ LacY β-Galactosidase Breaks lactose into glucose and galactose LacY Permease Actively transports lactose across cell membrane LacA Transacetylase Function unknown

Lac operon cont When lactose is not present, there is a very low level of transcription of these genes When lactose is present, rate of transcription increases 1,000x (in a matter of minutes) Negative inducible operon Negative – regulator gene inhibits transcription Inducible – normally in “off” position

Lac operator Overlaps 3′ end of promotor and 5′ end of first structural gene (lacZ)

Trp operon Contains 5 structural genes for 3 enzymes required for tryptophan synthesis 2 enzymes are composed of two polypeptide chains Negative – regulatory protein is a repressor Repressible – normally in “on” position

Trp operon cont Repressor is produced in an inactive form The repressor is unable to bind to the operator; RNA polymerase can bind to promotor, so transcription occurs When levels of tryptophan is high, it binds to the repressor, activating it Repressor can now bind to operator, blocking attachment of RNA polymerase

Riboswitches Sequences of mRNA that serve as potential binding sites for regulatory proteins Determines whether translation can occur or not

Ribozymes RNA molecule that is capable of acting as a biological catalyst (enzyme) Induced self-cleavage prevents translation