TRP operon

Gene regulation is important because it helps conserve energy - won’t waste energy by producing something that is not needed

Structural genes = produces proteins that are responsible for the structure/function of a cell (e.g. enzymes, receptors, hormones) and are usually found downstream
Regulatory genes = responsible for the production of regulatory proteins (e.g. repressor proteins, activator proteins) and can usually turn transcription on/off and are found upstream

\ \

A cluster of linked genes that share a common promoter and operator that are transcribed at the same time
Usually found in prokaryotes - such as the trp operon

The trp operon is a series of genes that are involved in the production of the amino acid tryptophan (trp)
Trp can then be used to produce proteins
The operon consists of:

1. A set of structural genes 2. An operator site (a site which regulates transcription) 3. A regulator gene that codes for a repressor

There are two ways in which the trp operon regulates gene expression- via repression of transcription, or via attenuation (the process that makes something weaker)
The repression model relies on the trp amino acid molecule acting as a co-repressor (trp acts as a negative control because the bound repressor prevents transcription)
The attenuation model depends on the fact that ribosomes attach to mRNA as it is being transcribed (translation begins before transcription of the whole mRNA molecule is complete)

\ \

The regulatory gene is always expressed - this produces a repressor protein
When trp levels are HIGH, trp will bind to the repressor to cause a conformational change, which allows the activated repressor to bind to the operator that is upstream of the structural genes responsible for trp production
This means that the RNA polymerase cannot proceed to transcribe those structural genes
TRANSCRIPTION OF GENES IS BLOCKED, NO PRODUCTION OF TRP CAN OCCUR, ENERGY IS SAVED

\ \

Remember, the regulatory gene is always expressed, so there are still repressor proteins floating around
When trp levels are LOW, there is no trp present to be able to bind to the repressor
Repressor remains inactive, cannot bind to operator, cannot block transcription
RNA polymerase can then proceed to transcribe the trp producing structural genes
TRANSCRIPTION CAN OCCUR, STRUCTURAL GENES ARE TRANSCRIBED, TRP PRODUCING PROTEINS ARE TRANSLATED AND MORE TRP IS MADE.

Upstream of the trp structural genes is a ‘leader sequence’, which encodes a 14-amino acid leader peptide containing two trp molecules
The function of this leader sequence is to control the trp operon based on trp availability
The leader sequence has 4 regions, that can form two different ‘hairpin’ secondary structures
If region 3 and 4 pair up, they form a loop-like structure called an attenuator (will terminate transcription)
If region 3 and 2 pair up, the attenuator does NOT form (transcription can continue)

When trp is low * When trp is low, the ribosome pauses at the two trp codons (not enough trp to add to the growing polypeptide chain), resulting in regions 2 and 3 pairing up - forms the anti-terminator * If regions 2 and 3 are paired up, that means that region 3 cannot pair with region 4 * Thus, transcription continues to the end of the trp operon and the bacteria can produce more trp
When trp is high

When trp is high, transcription does not stop at the 2 trp codons, it continues until it reaches a stop codon that is located between regions 1 and 2
The position of this stop codon means that region 2 cannot pair with region 3 - meaning that region 3 will pair with region 4 - this forms the transcription terminator
Therefore, only the leader peptide is translated
The remainder of the structural genes will no longer be transcribed or translated, because trp is not needed