Histidine genes, mapping

Fig.l. Gene maps of recombinant plasmids pTOH, pTTOH, pEOH, and pETOH. Abbreviations Pnc, trc promoter Px7, T7 and lac hybrid promoter Tat, twin-arginme TorA signal sequence of TMAO reductase OPH, organophosphoms hydrolase gene Hisg, hexa-histidine affinity tag. 

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. 

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). 

If the diploid has non-mutant (wild-type) phenotype, and it can be shown that they map at different positions (random spore analysis is most often used) then the lesions are in different genes. Such genes are named by adding a number after the letter code, e.g. his, hisl. Different genes which influence the same phenotype often produce proteins controlling for example a biosynthetic pathway. In this way the pathway and enzymes involved, in for example, histidine biosynthesis were elucidated. 

A. Mapping of the Histidine Enzyme Genes—Discovery of the Operon 360... 

Fio. 3. Genetic map of histidine mutations in Salmonella. The genes specifying the histidine enzymes, hisA through hisi, are in a cluster near 6 o clock on the map. HisR, U, W, S, O, and T are regulatory genes (see Section IV). HisP is involved with histidine trans port. This map is adapted from Fink and Roth [42]. 

Bacteria with regulatory mutations mapping in the hisS gene have been found to have a defective histidyl-tRNA synthetase [19,98]. The finding that synthetase mutants are derepressed indicates that histidine by itself does not act in the repression mechanism, since a reduced ability of this enzyme to esterify histidine to transfer RNA should if anything cause a buildup of the free histidine pool. Instead, histidine must at least bind to histidyl-tRNA synthetase for repression to occur the enzyme-histidine complex could be the repressor, or aminoacylated tRNA could be required. Earlier work of Schlesinger and Magasanik [10] had also indicated that histidine must pass beyond the free state... 

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. 

Fig, 26, General map of the histidine operon from data of Ames and Hartmann and collaborators (Ames, 1965), Capital letters between diagrams refer to gene loci. 

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