operons

Examples of operons in the following topics:

• The lac Operon: An Inducer Operon

  • The lac operon is an inducible operon that utilizes lactose as an energy source and is activated when glucose is low and lactose is present.
  • A major type of gene regulation that occurs in prokaryotic cells utilizes and occurs through inducible operons.
  • The lac operon is a typical inducible operon.
  • However, for the lac operon to be activated, two conditions must be met.
  • Describe the components of the lac operon and their role in its function

• The trp Operon: A Repressor Operon

  • The trp operon is a repressor operon that is either activated or repressed based on the levels of tryptophan in the environment.
  • These five genes are next to each other in what is called the tryptophan (trp) operon .
  • The five coding regions for the tryptophan biosynthesis enzymes are arranged sequentially on the chromosome in the operon.
  • The five genes that are needed to synthesize tryptophan in E. coli are located next to each other in the trp operon.
  • Explain the relationship between structure and function of an operon and the ways in which repressors regulate gene expression

• The AraC Regulator

  • The L-arabinose operon, also called ara operon, encodes enzymes needed for the catabolism of arabinose to xylulose 5-phosphate.
  • The L-arabinose operon, also called ara operon, is a gene sequence encoding enzymes needed for the catabolism of arabinose to xylulose 5-phosphate, an intermediate of the pentose phosphate pathway.
  • The operon is found in Escherichia coli (E. coli).
  • The ara operon is regulated by the AraC protein.
  • This complex is needed for RNA polymerase to bind to the promoter and transcribe the ara operon.

• Catabolite Activator Protein (CAP): An Activator Regulator

  • When glucose levels decline in E. coli, catabolite activator protein (CAP) is bound by cAMP to promote transcription of the lac operon.
  • This type of process can be seen in the lac operon which is turned on in the presence of lactose and absence of glucose.
  • When glucose levels decline in the cell, accumulating cAMP binds to the positive regulator catabolite activator protein (CAP), a protein that binds to the promoters of operons that control the processing of alternative sugars, such as the lac operon.
  • In these operons, a CAP-binding site is located upstream of the RNA-polymerase-binding site in the promoter.
  • As cAMP-CAP is required for transcription of the lac operon, this requirement reflects the greater simplicity with which glucose may be metabolized in comparison to lactose.

• Regulation by Biosynthetic Enzymes

  • Attenuation (in genetics) is a proposed mechanism of control in some bacterial operons that results in premature termination of transcription.
  • Attenuation is an ancient regulatory system, prevalent in many bacterial species providing fast and sensitive regulation of gene operons and is commonly used to repress genes in the presence of their own product (or a downstream metabolite).
  • A high level of tryptophan will permit ribosomes to translate the attenuator sequence domains 1 and 2, allowing domains 3 and 4 to form a hairpin structure, which results in termination of transcription of the trp operon.
  • Therefore, the rest of the operon will be transcribed and translated, so that tryptophan can be produced.
  • The attenuator sequence has its codons translated into a leader peptide, but is not part of the trp operon gene sequence.

• Planctomycetes

  • A number of essential pathways are not organized as operons , which is unusual for bacteria.
  • In genetics, an operon is a functioning unit of genomic DNA containing a cluster of genes under the control of a single regulatory signal or promoter.

• Attenuation

  • Attenuation is a regulatory mechanism used in bacterial operons to ensure proper transcription and translation.
  • An example of attenuation is the tryptophan operon.

• Control of Transcription in Archaea

  • Other characteristic archaean features are the organization of genes of related function—such as enzymes that catalyze steps in the same metabolic pathway into novel operons, and large differences in tRNA genes and their aminoacyl tRNA synthetases.

• Nonribosomal Peptide Antibiotics

  • The NRPS genes for a certain peptide are usually organized in one operon in bacteria and in gene clusters in eukaryotes.

• Genetics and Regulation of N2 Fixation

  • Nitrogen fixation is regulated by nif regulon, which is a set of seven operons which includes 17 nif genes.