Gene Regulation Packet #22

Prokaryotic vs. Eukaryotic Why different? Bacterial cells grow rapidly and have a relatively short life span Eukaryotic have a long life span and because of that, they have the ability to respond to many different stimuli Single gene is regulated in different ways in different cell types Although transcriptional-level control is important, control at other levels of gene expression is also important

Transcriptional Regulation Prokaryotes Most regulated genes in bacteria are organized into operons. Operon Gene complex consisting of a group of structural genes with related functions, plus the closely linked DNA sequences responsible for controlling them. Set of adjacent structural genes whose mRNA is synthesized in one piece, plus the adjacent regulatory signals that affect transcription of structural genes

Operon Each operon has a single promoter region upstream from the protein-coding region The operator serves as the regulatory switch for transcriptional-level control of the operon. Repressor Protein Binds to the operator sequence Prevents transcription Although RNA polymerase binds to the promoter, it is blocked from transcribing he structural genes When repressor is not bound, transcription proceeds

Inducible Operon The Lac Operon Operon that is normally turned off Repressor protein is synthesized in an active form that binds to the operator If lactose is present, the repressor is converted to allow lactose to bind at an allosteric site on the repressor protein changing its shape. The altered repressor cannot bind to the operator and the operon is transcribed.

Lac Operon II Positive vs. Negative Control Repressible, and inducible operons are under negative control. When the repressor protein binds to the operator Transcription of the operon is turned off. Positive Control Separate protein binds to the DNA and stimulates transcription CAP activates the lactose operon Requires the use of cAMP. Levels of cAMP increase as levels of glucose decrease. CAP binds on the promoter region and stimulates the binding of RNA polymerase.

Repressible Operon Trp Operon Operon that is normally turned on. Repressor protein is synthesized in an inactive form Cannot bind on the operator. The end product acts as a corepressor. When levels are high, corepressor binds at the allosteric site of the repressor Repressor changes shape Repressor binds on the operator Transcription ends.

Constitutive Genes These genes are neither repressible, nor inducible, but are active all times. Housekeeping gene CAP (catabolite activator protein) are produced constitutively These proteins work by recognizing and binding to specific base sequences in the DNA. The activity of constitutive genes, responsible for the production of CAP’s, is controlled by how efficiently RNA polymerase binds to their promoter regions

Gene Regulation in Eukaryotes Already covered in the transcription packet. Transcription factors

Homework Definitions Posttranscriptional controls DNA methylation Gene amplification Differential mRNA processing