Gene expression Regulatory networks in prokaryotes (L17)

Question 1

What is polycistronic mRNA?

Answer 1

mRNA that encodes for multiple different proteins (like in an operon)

Question 2

Describe the difference b/w

1. Inducible operons
2. Activator operons
3. Repressible operons

Answer 2

1. Inducible: default state is repressed, but inducer molecule will derepress it (eg lac operon)
2. Activator: default state is UNrepressed, but inducer molecule will activate it (by recruiting RNAP) (eg arabinose operon)
3. Repressible: default state is active, but corepressor molecule will repress it (and attenuation also occurs) (eg tryptophan operon).

Question 3

What are the protein products of the lac operon?

Answer 3

Gene Z: codes for ß-galactosidase which cleaves lactose-> glucose+galactose

Gene Y: codes for permease which causes cell to take up more lactose

Gene A: codes for transacetylase

Question 4

Why does the lac operon operator contain a palindromic sequence?

Answer 4

It is a very specific sequence that the lac repressor binds to, which makes it unlikely that it would randomly appear anywhere else in the genome.

The palindromic sequence allows for 2 repressor binding sites.

NOTE: the operator is the DNA sequence to which the repressor binds!

Question 5

What is special about Isopropylthiogalactoside?

Answer 5

IPTG is a allolactose mimic that induces transcription in the lack operon but it is never cleaved so it will forever remain induced.

Question 6

What are the 4 main DNA binding factor motifs?

Which is most common?

What do they do?

Answer 6

1. Zinc finger (most common)
2. Leucine-zipper
3. Helix-turn-helix
4. Helix-loop-helix

They are DNA binding proteins, so they would not be involved in translation (ie mRNA)

Question 7

What are the 4 scenarios of a lac operon w/ respect to glucose and lactose concentrations?

Answer 7

1. High Glc, High Lac: Operon derepressed (not much transcription)
2. High Glc, Low Lac: Operon fully repressed
3. Low Glc, High Lac: Operon activated (lots of transcription)
4. Low Glc, Low Lac: Operon repressed

Question 8

What is the relationship b/w Glucose and cAMP?

Answer 8

In the absence of glucose, cAMP lvls rise.

cAMP forms a complex with cAMP receptor protein (CRP) and the binding complex permits the binding of RNAP to the promoter.

Question 9

Describe attenuation in the repressible Trp operon when Trp levels are high.

What about when Trp lvls are low?

Answer 9

Not only does Trp act as its own corepressor and bind to the repressor which binds to the operator preventing and more transcription, but attenuation also occurs.

W/ high Trp lvls: In the leader sequence, the ribosome does not stall at the Trp codons and so the sequences 3&4 will form a hairpin loop and terminate transcription (which will end translation shortly after)

W/ low Trp lvls: The ribosome stalls at the Trp codons at the beginning of sequence 1, which leads to a hairpin loop at sequences 2&3 (not a termination structure) causing transcription to continue, and translation will follow after, creating Trp biosynthesis proteins.

Question 10

Do prokaryotes have histones?

Unrelated question: What makes attenuation possible in prokaryotes?

Answer 10

No;

The fact that ribosomes begin translating while RNA is transcribing at the same time.