I. THE HISTIDINE OPERON
A. Type of regulation
1. Attenuator regulation
a. An attenuator is a stem loop structure found in RNA that can cause RNA polymerase to stop transcription
1. Stem loop forms from complimentary sequences on the mRNA, causing an area of double stranded RNA containing a single-stranded loop
2. This structure causes RNA polymerase to pause, and then carry out its exergonic nuclease activity, degrading RNA (instead of polymerization), till it no longer is bound to DNA
a. Thus transcription is stopped
B. Overview
1. A bacterial cell does not want to waste the energy to synthesize enzymes to make an essential amino acid (histidine) if that amino acid is already present in the growth medium
a. The histidine operon is regulated to produce enzymes to make histidine only if histidine is absent
2. The absence of histidine causes the formation of the attenuator structure in the mRNA molecule
a. Many stem loop structures can be formed by the leader sequence of the mRNA
1. There are six sequences that are complimentary to the complimentary sequence on both their sides
a. Sequence 1 can bind to sequence 2
b. Sequence 2 can bind to sequence 1 or 3
c. Sequence 3 can bind to sequence 2 or 4
d. Sequence 4 can bind to sequence 3 or 5
e. Sequence 5 can bind to sequence 4 or 5
f. Sequence 6 can bind to sequence 5
b. One of these stem loops is an attenuator
1. The stem loop formed when regions 5 and 6 complimentary base pair is the attenuator
a. Prevents transcription from proceeding past the leader sequence
2. The attenuator stem loop forms when histidine is present
a. Hence enzymes for histidine synthesis will not occur
C. Mechanism
1. The region upstream from the complimentary sequences contains many consecutive histidine codons
a. If histidine is absent, the ribosome would stall here
1. The ribosome, stalled here, would prevent complimentary region one from being able to hybridize with complimentary region two
b. If histidine is present, the ribosome will be directly behind the RNA polymerase
1. This means that as region 2 is synthesized, it would be able to bind to region 1
2. When histidine is absent the attenuator is not formed
a. The ribosome stalls at the histidine codons near region 1
1. 1 is prevented from binding to 2
2. 2 binds to 3
3. 4 binds to 5
4. No attenuator
a. 6 cannot bind to 5, forming the attenuator, since 5 is with 4
3. When histidine is present the attenuator is formed
a. The ribosome does not stall at the his codons near region 1, since histidine is present
1. Sequence 1 can bind to sequence 2
2. Sequence 3 can bind to sequence 4
3. Sequence 5 can bind to sequence 6
a. This forms the attenuator stem loop
i. Caused RNA polymerase to fall off mRNA
b. This prevents the transcription of genes to make histidine
I. THE HISTIDINE OPERONA. Type of regulation1. Attenuator regulationa. An attenuator is a stem loop structure found in RNA that can cause RNA polymerase to stop transcription1. Stem loop forms from complimentary sequences on the mRNA, causing an area of double stranded RNA containing a single-stranded loop2. This structure causes RNA polymerase to pause, and then carry out its exergonic nuclease activity, degrading RNA (instead of polymerization), till it no longer is bound to DNAa. Thus transcription is stoppedB. Overview1. A bacterial cell does not want to waste the energy to synthesize enzymes to make an essential amino acid (histidine) if that amino acid is already present in the growth mediuma. The histidine operon is regulated to produce enzymes to make histidine only if histidine is absent2. The absence of histidine causes the formation of the attenuator structure in the mRNA moleculea. Many stem loop structures can be formed by the leader sequence of the mRNA1. There are six sequences that are complimentary to the complimentary sequence on both their sidesa. Sequence 1 can bind to sequence 2b. Sequence 2 can bind to sequence 1 or 3c. Sequence 3 can bind to sequence 2 or 4d. Sequence 4 can bind to sequence 3 or 5e. Sequence 5 can bind to sequence 4 or 5f. Sequence 6 can bind to sequence 5b. One of these stem loops is an attenuator1. The stem loop formed when regions 5 and 6 complimentary base pair is the attenuatora. Prevents transcription from proceeding past the leader sequence2. The attenuator stem loop forms when histidine is presenta. Hence enzymes for histidine synthesis will not occurC. Mechanism1. The region upstream from the complimentary sequences contains many consecutive histidine codonsa. If histidine is absent, the ribosome would stall here1. The ribosome, stalled here, would prevent complimentary region one from being able to hybridize with complimentary region twob. If histidine is present, the ribosome will be directly behind the RNA polymerase1. This means that as region 2 is synthesized, it would be able to bind to region 12. When histidine is absent the attenuator is not formeda. The ribosome stalls at the histidine codons near region 11. 1 is prevented from binding to 22. 2 binds to 33. 4 binds to 54. No attenuatora. 6 cannot bind to 5, forming the attenuator, since 5 is with 43. When histidine is present the attenuator is formeda. The ribosome does not stall at the his codons near region 1, since histidine is present1. Sequence 1 can bind to sequence 22. Sequence 3 can bind to sequence 43. Sequence 5 can bind to sequence 6a. This forms the attenuator stem loopi. Caused RNA polymerase to fall off mRNAb. This prevents the transcription of genes to make histidine
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