Trp Operon - Attenuation - Part II

Question 1

Describe the 1st step in Attenuation occurring

What is high in levels?

Answer 1

High levels of tryptophan is available in cell

Question 2

Describe the 2nd step in Attenuation occurring

What process begins?

Answer 2

• Process of transcription begins
• Almost immediately, translation begins in the same place

Question 3

Describe the 3rd step in Attenuation occurring

What happens to ribosome in Domain 1?

Answer 3

• Domain 1 is being transcribed.
• When ribosome is translating domain 1 it reaches the trp codons.
• Ribosome doesn’t stall as plenty of tryptophan is available and brought to ribosome through tRNA.

Question 4

Describe the 4th step in Attenuation occurring

Hairpin loop formation

Answer 4

• Ribsome begins to translate Domain 2.
• So Anti-terminator hairpin loop cannot form.
• Thus Domain 3 and 4 interact through self-complementary regions.
• Terminator hairpin loop is formed.

Question 5

Describe the 5th step in Attenuation occurring

Effect on transcription

Answer 5

• Formation of terminator hairpin loop causes RNA Polymerase to detach before reaching structural genes.
• Transcription ends.
• Ribosome unclamps and falls off.

Question 6

Describe the 6th step in Attenuation occurring

Aftermath

Answer 6

• Only an attenuated mRNA is made
• This does not encode for any tryptophan synthetase
• No more tryptophan is produced

Question 7

Describe the 1st step in Attenuation not occurring

Levels of tryptophan

Answer 7

Low levels of tryptophan present in cell

Question 8

Describe the 2nd step in Attenuation not occurring

What processes begin?

Answer 8

• Transcription begins as RNA polymerase binds to promoter region and moves down to Leader.
• Translation also begins in same place

Question 9

Describe the 3rd step in Attenuation not occurring

Ribosome reaches trp codons

Answer 9

• Domain 1 is being transcribed.
• When ribosome is translating domain 1 it reaches the trp codons.
• Ribosome stalls as little tryptophan is present.

Question 10

Describe the 3rd step in Attenuation not occurring

Hairpin loop formation

Answer 10

• Stalled ribosome remains translating Domain 1, waiting for tryptophan.
• RNA Polymerase finishes transcribing Domain 3.
• Domain 2 and 3 interact through self-complementary regions.
• Anti-terminator hairpin loop is formed.

Question 11

Describe the 3rd step in Attenuation not occurring

How RNA Polymerase can continue transcribing

Answer 11

• As Anti-terminator hairpin loop if formed terminator hairpn loop cannot form.
• Thus, RNA Polymerase doen’t detach, it continues transcribing structural genes for trp operon.

Question 12

Describe the 4th step in Attenuation not occurring

Aftermath

Answer 12

• Transcription is completed by RNA Polymerase.
• Translation is eventually finished to produce more tryptophan in cell.