5.5 Control Mechanisms There are approximately genes that exist to code for proteins in humans. – Not all proteins are required at all times. – Some genes are required at all times Housekeeping genes – Transcription factors turn on genes when required. Gene regulation can occur at four levels. – Transcriptional Regulates which genes are transcribed OR controls rate of transcription – Posttranscriptional mRNA undergoes changes in the nucleus. – Translational How often and how rapidly mRNA transcripts are translated into proteins. – Posttranslational Before many proteins become functional they must leave the cell.

Lac OPERON Lactose is a disaccharide composed of glucose and galactose. – E. coli use lactose for energy. Must split lactose into monomers. β-galactosidase When lactose is not present, no need for production. The gene for β-galactosidase is part of an operon. Comprised of a cluster of structural genes, a promoter, and a short sequence of bases between the two known as an operator

Lactose

The lac operon consists of a cluster of three genes that code for proteins in the metabolism of lactose. – lacZ, lacY, and lacA lacZ codes for β-galactosidase lacY codes for β-galactosidase permease lacA has an unknown function

Lac Operon

The LacI protein is a repressor protein that blocks the transcription of β-galactosidase gene by binding to the lactose operator and getting in the way of RNA polymerase. – When lactose is present, it binds to the LacI protein, allowing β-galactosidase to be transcribed. Lactose acts as a signal molecule or inducer

Trp OPERON Used to control Tryptophan metabolism Transcription is blocked when there are high amounts of tryptophan present. – The level of tryptophan is the effector. Consists of five genes. – Code for five polypeptides that make three enzymes needed to synthesize tryptophan.

Trp OPERON-a corepressor

5.5 - Summary

5.5 Vocabulary