Click the following line for an AP Bio-level tutorial about operons.

Scroll down below the video for the lyrics


Hey I’m E. coli a bacteria cell
Gonna tell you ‘bout something I do so well
It’s the way I turn my genes off and on
It’s a regulatory system called an operon

My chromosome has over 4 million bases
And 4000 genes so I need some basis
For deciding which gene gets activated.
And operons are how my genes are regulated

One operon has initials T-R-P
The trp operon, its function, you see,
Is coding for enzymes that make tryptophan,
An amino acid. Here’s my plan

If there’s tryptophan around,
then E coli shouldn’t make it so I shut the system down.
But if tryptophan is lacking and it’s something I require,
then I turn the system o  and make what I desire

But how does a bacterium ascertain
When and what to synthesize when it’s got no brain?Let’s examine the structure to get the information

‘Bout the workings of operonic gene regulation

Oper operons
How prokaryotes turn genes off and on
A cluster of genes transcribed as one RNA
Oper Operons
How bacteria turn genes off and on
Repressing, inducing, the operonic way

Let’s start with the structural genes which code
for enzymes and other proteins.
and these structural genes get transcribed as one
Big RNA transcript, we’ve just begun.

These structural genes | are under the control
of one promoter, the promoter’s role
Is to let RNA polymerase bind and read
The structural genes and transcribe

After the promoter’s an operator that
Controls if the genes are transcribed now or later
The operator’s where a repressor protein binds
If repressor’s attached, nothin’s transcribed

The repressor’s the product of a regulatory gene
Located away from this structural scene
The repressor has two binding sites
It functions like a switch for turning on a light.

Site 1’s for tryptophan when it’s occupied.
The repressor binds the operator on its other side.
But when site 1’s empty, side 2’s conformation
Won’t bind the operator’s location.

So when tryptophan is in the environment
It binds with the repressor makes the other side stick
to the operator, blocking transcriptiono8
The repressor’s on regulatory mission.

Think of the logic, the energy saved,
if tryptophan’s around E. coli shouldn’t make it.
But if tryptophan’s something E. coli can’t find,
The repressor changes shape and no longer binds.

So RNA polymerase transcribes |
the genes to make tryptophan that I require.
When I’ve made enough the tryptophan binds and effects
The repressor which changes shape and now connects

To the operator, blocking transcription.
It’s negative feedback, evolved to perfection
an adaptation, such efficiency
Come on class sing about operons with me!


The trp operon’s the repressible kind
Now let’s learn ‘bout the inducible kind.
This one’s called lac it involves lactose
A disaccharide in milk as everyone knows

Lac’s structural genes make proteins and enzymes
For hydrolyzing lactose to monosaccharides
But if lactose is absent I shut this down
‘Cause I only need these enzymes when lactose is around.

Lac’s the opposite of trp who we just met,
The default condition’s off with the repressor sitting
on the operator, blocking transcription,
So no enzymes are made for lactose digestion

When lactose is around it diffuses into me
And binds with the repressor which shifts beautifully
into a form where it can’t bind the operator,
Transcription starts like an escalator.

See how lactose is an inducer,
Inducing transcription —  a brilliant maneuver,
Then these enzymes break lactose down,
Until it’s digested, and no longer around.

And when it’s gone there’s no lactose to bind
with the repressor, which shape-shifts on the other side,
And binds the operator so transcription ends
It’s negative feedback again!

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