Histidine, operon

The histidine operon encodes the enzymes of the histidine biosynthetic pathway. It is advantageous to the cell to produce these enzymes when histidine is not available in the surroundings, but to turn off their synthesis when histidine is readily available. 

Figure 1-5-3. Attenuation Control of Transcription in the Histidine Operon...

Regulation of histidine synthesis. In all, ten different genes code for the enzymes of histidine biosynthesis in Salmonella typhimurium. They are clustered as the histidine operon, a consecutive series of genes which are transcribed into messenger RNA as a unit.250 251 The gene symbols His A, HisB, etc., are indicated in Fig. 25-13, and their positions on the E. coli gene map are indicated in Fig. 26-4. The gene HisB codes for a complex protein with two different enzymatic activities as shown in Fig. 25-13. 

For example, the leader peptide for the phenylalanine operon includes 7 phenylalanine residues among 15 residues. The threonine operon encodes enzymes required for the synthesis of both threonine and isoleucine the leader peptide contains 8 threonine and 4 isoleucine residues in a 16-residue sequence. The leader peptide for the histidine operon includes 7 histidine residues in a row. In each case, low levels of the corresponding charged tRNA causes the ribosome to stall, trapping the nascent mRNA in a state that can form a structure that allows RNA polymerase to read through the attenuator site. 

Figure 31.36. Leader Peptide Sequences. Amino acid sequences and the corresponding mRNA nucleotide sequences of the (A) threonine operon, (B) phenylalanine operon, and (C) histidine operon. In each case, an abundance of one amino acid in the leader peptide sequence leads to attenuation.

C) histidine operon. In each case, an abundance of one amino acid in the leader peptide sequence leads to attenuation. 

The histidine genes of S. typhimurium are among the best-characterized operons. The structural genes for the enz5mies of the histidine operon (ten enzymes that convert the 5-carbon chain of phosphoribosyl pyrophosphate to histidine) are in a cluster on the Salmonella chromosome. More than a thousand histidine-requiring mutants have been located on a fine-structure map of the operon (Whitfield et al. 1966). 

Regulation of the Histidine Operon Covalent Modification of Phosphofruc-... 

Stephens JC, Am SW, Ames BN. Guanosine S -diphosphate 3 -diphosphate (ppGpp) Positive effector for histidine operon transcription and general signal for amino acid deficiency. Proc Natl Acad Sci USA 1975 72 4389-4393. 

A rough estimate of the size of the histidine operon is obtained from the sedimentation velocity of the polycistronic messenger. In Martin s experiment [48], a sedimentation coefficient of 34 S was found. A more recent experiment by Venetianer et a , confirmed this figure [50]. The value of 34 S corresponds to a length of about 10,000 nucleotides in the mRNA. 

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