Operon – Coordinately Regulated Units of Gene Expression

Operon – Coordinately Regulated Units of Gene Expression

  • The DNA of prokaryotes is organized into a circular chromosome supercoiled in the nucleoid region of the cell cytoplasm.
  • Functionally related genes in bacterial cells are frequently clustered together (separated only by a few base pairs) in the chromosome as a single transcriptional unit termed an operon.

(Example: All of the genes needed to use lactose as an energy source are coded next to each other in the lac operon).

Distribution in the nature

  • Operons occur primarily in prokaryotes but also in some eukaryotes, including nematodes such as C.elegans and the fruit fly, Drosophila melanogaster.
  • rRNA genes often exist in operons that have been found in a range of eukaryotes including chordates
  • Operons can also be found on bacteriophages (bacteria infecting viruses).

General structure (components of an operon)

An operon is made up of 3 basic DNA components: promoter and operator (regulatory sequences) and structural genes (set of genes that are co regulated).

Figure 1: The organization of a typical operon

1) Promoter

  • The promoter is the DNA sequence of the operon recognized by DNA-dependent RNA polymerase.
  • The initiation site for RNA synthesis is immediately downstream of the promoter.
  • Transcription is initiated at promoters located just upstream (5’) from the coding regions of structural genes.

2) Structural genes

  • Structural genes with related functions in an operon are transcribed together under the control of a single promoter.
  • The mRNAs of operons consisting of more than one structural gene are polycistronic (a single mRNA molecule that codes for more than one protein). .
  • For example, the tryptophan operon mRNA of E. coli contains the coding sequences of five different genes (TrpA, TrpB, TrpC, TrpD and TrpE). Because they are co-transcribed, all structural genes in an operon are coordinately expressed.

Transcription of a set of contiguous structural genes is regulated by two controlling elements: operator and regulatory genes (not a part of operon)

 3) Operator

  • The operator is the region of DNA of the operon that is the binding site for the regulatory protein called repressor.
  • Operator regions are contiguous with promoter regions. Sometimes operators and promoters even overlap, sharing a short DNA sequence.
  • Operator regions are often located between the promoters and the structural genes that they regulate.
  • The operator usually overlaps the 3’ end of the promoter and sometimes the 5’ end of the first structural gene.

How repressors regulate the activity of operon?

  • When a repressor is bound to the operator, it sterically prevents RNA polymerase from transcribing the structural genes in the operon.
  • When the operator is free of repressor, RNA polymerase can transcribe structural genes in the operon.

Whether the repressor will bind to the operator and turn off the transcription of the structural genes in an operon is determined by the presence or absence of effector molecules.

Regulatory gene

  • A regulator gene helps to regulate the transcription of the structural genes of the operon.
  • The regulatory gene codes for a regulatory protein called repressor. This gene is constantly expressed.
  • The regulator gene is not considered part of the operon, although it affects operon function.
  • The regulatory gene does not need to be in, adjacent to, or even near the operon to control it.
  • The regulator gene has its own promoter and is transcribed into a relatively short mRNA, which is translated into a small protein.
  • This regulator protein bind to a region of DNA called the operator and affect whether transcription can take place.

 Example: The lac repressor (the regulatory protein of the lac operon) encoded by the lac I gene

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