The Chapter 15 Homework is due Wednesday, January 30 at 11:59 pm. The Chapter 15 Test is Thursday, January 31 There will be a POGIL Quiz tomorrow

Overview of Prokaryotic Gene Regulation Chapter 15 Overview of Prokaryotic Gene Regulation

You Must Know A regulatory gene is a sequence of DNA that codes for a regulatory protein such as a repressor protein. How the components of an operon function to regulate gene expression.

Prokaryotic Gene Regulation Natural selection has favored bacteria that produce only the gene products needed by the cell.

A cell can regulate the production of enzymes by feedback inhibition or by gene regulation.

Polypeptide subunits that make up enzymes for doing a particular task Gene expression in bacteria is controlled by operons. Operons are a group of functionally related genes that can be coordinately controlled by a single “on-off switch”. The regulatory “switch” is a segment of DNA called an operator usually positioned within the promoter. operon Promoter Operator RNA polymerase Stop codon An operon is the entire stretch of DNA that includes the operator, the promoter, and the genes that they control. . Start codon E D C B A Polypeptide subunits that make up enzymes for doing a particular task 6

Polypeptide subunits that make up enzymes for doing a particular task The operon can be switched off by a protein repressor. Promoter Regulatory gene Polypeptide subunits that make up enzymes for doing a particular task E D C B A RNA polymerase mRNA Promoter operon Genes of operon Operator mRNA Protein Inactive repressor The repressor prevents gene transcription by binding to the operator and blocking RNA polymerase. The repressor is the product of a separate regulatory gene. 7

The repressor can be in an active or inactive form, depending on the presence of other molecules. DNA mRNA Protein Active repressor No RNA made (corepressor) A corepressor is a molecule that cooperates with a repressor protein to switch an operon off. 8

Tryptophan Tryptophan Operon Promoter Genes of operon DNA Regulatory mRNA Protein RNA polymerase Inactive repressor Operator Start codon Stop codon mRNA 5 trpE trpD trpC trpB trpA E D C B A (a) Tryptophan absent, repressor inactive, operon on Polypeptide subunits that make up enzymes for tryptophan synthesis 5 3 trpR Promoter Genes of operon RNA polymerase Operator Start codon Stop codon mRNA 5 trpE trpD trpC trpB trpA E D C B A Polypeptide subunits that make up enzymes for tryptophan synthesis trp operon DNA mRNA Protein Active repressor No RNA made Tryptophan (corepressor) (b) Tryptophan present, repressor active, operon off By default the trp operon is on and the genes for tryptophan synthesis are transcribed. When tryptophan is present, it binds to the trp repressor protein, which then turns the operon off. The repressor is active only in the presence of its corepressor tryptophan; thus the trp operon is turned off (repressed) if tryptophan levels are high. Tryptophan 9