The lactose operon structure was mentioned earlier, and the metabolic pathway of lactose in Escherichia coli was described in detail, but it was found that the operon structure of histidine in the prokaryotic metabolic pathway is very similar to the longitudinal structure of lactose metabolism, first of all, we will introduce the operon structure of histidine, which contains two promoters and a manipulative gene structure (PO), between which there are genes encoding imidazopyruvase (I) and glutamate transmethrosinase (G) and regulatory genes (C), After the two PO structures, there are genes encoding uroglycase (U) and histidine aminolyase (H), which is the operon structure of histidine metabolism, let's first talk about how histidine is metabolized, first it becomes uroglycine under the action of H enzyme, and then becomes imidazole pyruvate catalyzed by U enzyme, and then becomes iminoglutamate under the action of I enzyme, and finally becomes glutamate amine under the action of G enzyme!
When histidine is present, the metabolic intermediate product of histidine uricotide binds to the repressor protein encoded by its C regulatory gene in the form of a signaling molecule, so that the repressor protein cannot bind to the manipulative gene, and then the related genes of its metabolism are transcribed, while on the contrary, when histidine is not present, its urotide content is extremely low, resulting in the repressor protein binding to DNA and inhibiting the occurrence of transcription. In positive regulation, when the carbon source in the medium is lacking, as mentioned earlier, ATP can be directly metabolized into CAMP through related enzymes, so as to bind to the activation protein CAP encoded by the regulatory gene, and act on its cap site to promote RNA polymerase for transcription, but there is also a situation here that when the nitrogen source is lacking, the cell will produce a lack of signal to stimulate glutamine synthetase and activate it, so that this enzyme acts on RNA polymerase to promote transcription, if there is histidine, This will make histidine metabolism more efficient.
First of all, let's still introduce the operon structure of its regulation, which has the regulatory gene R and promoter P and the regulatory gene O, and then the structural gene (EDCBA) encoding related proteins, and what is fascinating is the attenuator structure between the structural gene and the manipulator gene, which is composed of the leading region and the attenuator subregion, which will be detailed in the regulation of transcription speed in the future.
First of all, let's talk about the negative regulation of tryptophan synthesis pathway, the biggest difference between it and lactose metabolism and histidine metabolism pathway is that the repressor protein encoded by the regulatory gene R is inactive, but when tryptophan exists, the repressor protein can bind to tryptophan, so that the repressor protein concept changes and binds to the O gene, so that transcription stops, and when tryptophan content is very low, transcription begins, and tryptophan at this time is called corepressor. Let's focus on the very interesting regulation of attenuator structures, which should have been transcribed normally when cells contain trace amounts of tryptophan, but it has been found that RNA polymerases move the leader region out of DNA-RNA.
After sequencing the leading region, people found that there are four inverted repeating sequences (palindromic sequences) in it, which are 60 68 (1 83 (2 121 (3 134 (4), and after 4 is a U-rich sequence, and 28-68 is the ORF box, which can encode a short peptide, because there is a stop codon UGA after 1, and the short peptide is composed of 14 amino acids, called the leader peptide, It actually contains two tryptophans, accounting for a ratio of 1 7, which shows that it is a leading peptide rich in tryptophan, and the aforementioned reverse repeat can synthesize the secondary stem-loop structure of mRNA, and due to the spatiotemporal synchronization of prokaryotic transcription and translation.
At this time, it is interesting that when the tryptophan raw material is sufficient, the translation speed is significantly accelerated, the ribosome is an RNA-large protein complex, and its moving speed is very fast, immediately occupying the 1st region and part of the 2nd region, forcing the 3rd and 4th regions to form a stem-loop structure, and there is a U sequence behind it, which becomes the attenuator structure of transcription termination, which is the so-called transcriptional termination of independent factors, and on the contrary, when the tryptophan raw material is insufficient, the ribosome moves extremely slowly and stagnates in the 1st region. At this time, the 2nd and 3rd regions that happened to be released became the stem-loop structure, and the 3rd region was followed by the 4th region, instead of a U sequence, so that there was no attenuator structure in the leading region of the subsequent mRNA sequence, and the RNA polymerase could be transcribed to regulate tryptophan synthesis.
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