The Histidine Operon

The study of orientation of the histidine operon on the genetic map of S. typhimurium (Hartmann et al., 1965) showed that if this operon is located on the bacterial chromosome, it lies in a clockwise direction between the following genes (operons) gal—try - 1-HisO-G-D-C B- H A-F I-E-metG-purG-str-metA. 

It is usually assumed that the histidine operon is transcribed into one molecule of messenger RNA, which is then transcribed by the polyribosome system from one end to the other. The histidine operon and corresponding mRNA consist of about 13,000 nucleotides. The possibility that such a polycistronic histidine messenger RNA exists was demonstrated by the work of Martin (1963). The RNA fraction, labeling of which took place differently in histidine-constitutive and histidine-deficient strains, had a sedimentation 

General map of the histidine operon from data of Ames and Hartmann and collaborators (Ames, 1965), Capital letters between diagrams refer to gene loci. 

Stages of reactions successively controlled by each gene are circled. Usual names of enzymes are given. Below each name of an enzyme its approximate maximal molecular weight (x 10 ) is given. Extent of certain mutations (deletions) is shown at the bottom of the diagram. 

Whitfield and co-workers (1964) determined the molecular weight of nearly all the enzymes of the histidine operon. Adding these weights together and allowing for the dimerism of some enzymes, they calculated that the histidine operon must code the position of 4500 amino acid residues. If the size of the operon (13,000 nucleotides) is taken into account, the code ratio (nucle-otides/amino acid) in this case is 2.9/1, in good agreement with the theoretical ratio of 3/1 for a triplet code. 

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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. 

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-... 

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. 

In Section III,B, the phenomenon of polarity was discussed in relation to its implications for a polycistronic mRNA for the histidine operon. For the deduction of the presence of a polycistronic mRNA, it was not necessary to know the specific mechanism by which polar mutations exerted their effect. However, a great deal of effort has been expended in trying to understand this mechanism, and these studies have produced a greater understanding of the translation process. 

The gene order in the histidine operon does not correspond to the order of the reactions in the pathway. It is possible that the order is simply one of chance, but one theory [47] suggests that ordering in... 

In an attempt to study natural polarity, mutations in different parts of the histidine operon were isolated which cause polarity, but do not result in a histidine requirement [70]. The very existence of these modulation mutants shows that it is possible for the cell to optimize the gradient of production of the enzymes of an operon. The analysis of one of these mutants is discussed in the following section. 

Fig. 4. Patterns of derepression of the histidine operon, redrawn from Marver et al. [77]. Histidine was made growth-limiting at time zero. A, simultaneous derepiession B, sequential derepression C, sequential derepression in the presence of ribosyl-AIC of a deletion mutant lacking parts of the hisB and hisH genes.

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