UNIT 3: Lecture 16->REGULATION OF GENE EXPRESSION IN OPERONS- BIOLOGICAL INFO FLOW

OPERONS

Regulation of gene expression
OPERONS

Genes organized in tandem along DNA and transcribed as a single unit from one promoter resulting in a polycistronic mRNA.

Regulation of gene expression
Bacterial cells only need to synthesize the (…) in the … & … operon IF those sugars are present.

Bacterial cells only need to synthesize the proteins in the lac & mal operon IF those sugars are present.

QUESTION ABOUT BACTERIA AND GENE EXPRESSION

Bacteria was transferred to nutrient medium containing lactose. B-galactosidase is an enzyme that E. coli uses to break down lactose.

When lactose is present, B-galactosidase goes up in time.

Prokaryotes organize some open reading frames in clusters = operons

A set of coding sequences for related proteins, all sharing the same promoter and terminator

operons
What is the result of the transcription of an operon?

One long mRNA, encoding multiple proteins.

Operator

A region of DNA where a regulatory protein binds. It sometimes overlaps a bit with the promoter.

Characteristics of operons in prokaryotes

• Each ORF (open reading frame) in the mRNA encoding one polypeptide has its own upstream ribosome-binding site.

• If one ORF has a mutation that results in an early stop codon, the other ORF can still be translated.

• RNA polymerase transcribes a single mRNA containing multiple ORFs.

Operons
Where can we find the OPERATOR (regulatory region)?

Either upstream or downstream, (and sometimes overlapping) the promoter. The regulatory protein binds here.

Regulatory proteins can bind to DNA to activate or repress gene expression.

Which are the two types of gene regulation?

Positive regulation and Negative Regulation

Positive Regulation

• Activator protein binds by the promoter (operator) and increases transcription.

• RNA polymerase binds

• Transcription

• ex: MalT

Negative regulation

• repressor protein binds to a region by the promoter (operator) and decreases transcription

• no RNA polymerase binds

• No transcription

• Ex: LacI

Regulation of gene expression

Two examples of environmentally regulated genes

lac and mal operons → catabolic (break down) operons

-bacterial cells metabolize sugars

-some cells use lactose or maltose as a source of energy and carbon

-The cells ONLY need to synthesize the proteins in the lac or mal operons IF those sugars are present.

lac Operon - in bacteria

Which are the Genes and Proteins in the lac operon?

Genes

• 3or 4 letter abbreviation (no caps) written in italics: lacI, lacZ, lacY, lacA

Proteins (= gene “product”)

• 3 or 4 letter abbreviation with caps: LacI, LacZ, LacY, LacA

Proteins (= gene “product”) in lac Operon

• 3 or 4 letter abbreviation with caps: LacI, LacZ, LacY, LacA

1. LacZ protein: breaks down lactose into glucose and galactose

2. LacY protein: transport protein that transports lactose into the cell.

3. LacA protein: not to be necessary for lactose metabolism

What does it mean to have basal levels of expression?

Very low or minimal levels of expression.

lac Operon - When does the cell need to express the LacZ, LacY, and LacA enzymes?

When there is lactose in the cell.

lac Operon

The regulation of the lactose operon:
LacI is a positive/negative regulatory protein of the lac operon.

The promoter for the lac operon is weak/strong, and binds sigma/RNA polymerase strongly/weakly.

The signal molecule is:

LacI is a negative regulatory protein of the lac operon.

The promoter for the lac operon is strong, and binds sigma/RNA polymerase strongly.

The signal molecule is: lactose.

lac Operon Regulation

lac Operon Regulation

mal operon

Which are the Genes and Proteins in the mal operon?

Genes

• 3 or 4 letter abreviation (no caps) written in italics: malP, malQ, malT

Proteins (=gene “product”)

• 3 or 4 letter abreviation with caps: MalP, MalQ, MalT

• Different genes/proteins involved in the same cellular process often give the same 3 letter abbreviation.

mal operon
MalP & MalQ→ proteins involved in degrading:

MalP & MalQ→ proteins involved in degrading:maltose to glucose

When does the cell need to express the MalP and MalQ enzymes?

When there is maltose in the cell

mal operon
MalT is a positive or negative regulator?

MalT is a positive regulator.

It is a separate transcription unit.

Gene malT is constitutively expressed → MalT protein is made all the time but in small amounts.

mal operon
Is the promoter for malPQ a strong or weak promoter?

The promoter of malPQ is very weak, it initiates very little to almost no transcription.

mal operon→ ABSENCE OF MALTOSE

mal operon→ PRESENCE OF MALTOSE

SUMMARY mal operon

The regulation of the maltose operon:

MalT is a positive/negative regulatory protein of the malPQ operon.

The promoter for the malPQ operon is weak/strong, and binds sigma/RNA POL strongly/weakly.

The signal molecule is:

The regulation of the maltose operon:

MalT is a positive regulatory protein of the malPQ operon.

The promoter for the malPQ operon is weak, and binds sigma/RNA POL weakly.

The signal molecule is: maltose.

LEVELS OF GENE EXPRESSION

1. high levels

2. basal levels

3. zero levels

*RECALL: the gene fr regulatory proteins are expressed constitutively—at lower levels, compared to high, these are expressed all the time.

LEVELS OF GENE EXPRESSION

1. high levels

Gene/operon is transcribed at high levels.

ex: RNA POL (& transcription factors) bind strongly/tightly and often to the promoter to initiate transcription.

LEVELS OF GENE EXPRESSION

2. Basal levels

Gene/operon is minimally (sometimes rarely) transcribed. i.e. RNA Pol binds weakly and rarely to the promoter and transcription is not initiated very often.

LEVELS OF GENE EXPRESSION
3. Zero Levels

No transcription/not possible. Not functional. Usually due to some deletion or mutation (ex: promoter deletion) that prevents transcription from initiation.

QUESTION FOR GENE EXPRESSION:

Lac Operon Regulation, ABSENCE of lactose, transcription level of lacZYA genes?

Basal levels, the operon is still functional, not perfect binding. Repressor can bind from time to time, permitting a little of transcription.

Lac Operon Levels of gene expression

LEVELS OF EXPRESSION

high and low levels in mal and lac operons

High levels

lac operon - when lactose is present

mal operon - when maltose is present

Low/basal levels

lac operon - when lactose is absent

mal operon - when maltose is absent

Signal Molecules→ Inducers vs Co-repressors

Catabolic operons

Signal molecules

Signal Molecules→ Inducers vs Co-repressors
Catabolic operons

ex: mal and lac operons

=produce proteins that are involved in catabolism (breakdown) of the signal molecule

Signal Molecules→ Inducers vs Co-repressors
Signal Molecule

ex: lactose and maltose

=if the operons are expressed (transcribed) in the presence of the signal molecule (regardless of negative or positive regulation, the signal molecule is an inducer.

Signal Molecules→ Inducers vs Co-repressors

Catabolic operons

ex: mal and lac operons

=produce proteins that are involved in the catabolism (breakdown of signal molecule)

Signal Molecules→ Inducers vs Co-repressors

Signal Molecule

ex:lactose or maltose

=if the operons are expressed (transcribed) in the presence of the signal molecule (regardless of negative or positive regulation), the signal molecule is an inducer.