| Back |
- operon - group of genes under control of same regulatory mechanism and expressed at same time, more common in prokaryotes so genomes smaller, and efficient as all genes involved in prod of product (not needed) turned off
- glucose easier to metbaolise so main respiratory substrate, but lactose alternative in short supply of glucose and diff enzymes needed to metabolise it
- The lac operon is group of 3 structural genes (lacZ, lacY and lacA, structural as code for 3 enzymes transcribed onto mRNA; B-galactosidase, lactose permease and transacetylase) involved in metabolism of lactose
- a regulatory gene (lacI) is located near operon and codes for repressor protein (prevents transcription of structural genes in lactose absence), repressor protein binds to operator which prevents RNA polymerase binding to promotor and starting transcription (down regulation), glucose presence dec cAMP levels, so CRP cant work and transcription dec (so less enzymes and less lactose broken down)
- when lactose present, it binds to receptor protein (which changes shape and no longer complimentary to operator) so now RNA polymerase binds to promotor and transcription occurs to form the enzymes, when lactose present, cAMP receptor protein (CRP) binds to RNA polymerase, but only up-regulates transcription (inc rate/efficiency) when bound to cAMP
- if both lactose and glucose present, glucose still metabolised (as prefered substrate)
|