Threonine operon

May JJ, Wendrich TM, Mahariel MA (2001) The dhb Operon of Bacillus subtilis Encodes the Biosynthetic Template for the Catecholic Siderophore 2,3-Dihydroxybenzoate-glycine-threonine Trimeric Ester Bacillibactin. J Biol Chem 276 7209... 

E. coli The whole L-threonine operon was amplified by plasmid-based overexpression in E. coli L-threonine producer A threefold increase in L-threonine production 13.4 Miwa et al. [48]... 

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.

Figure 31.35 Leader peptide sequences. Amino acid sequences and the corresponding mRNA nucleotide sequences of the (A) threonine operon.

Figure 7. Logarithmic gain for the repressible threonine biosynthetic operon of Escherichia coli. An auxotrophic mutant of E. coli B unable to synthesize homoserine was grown for more than four generations in glucose minimal medium supplemented with various concentrations of homoserine (Savageau and Steward, 1970). At low concentrations of homoserine, a correspondingly low amount of threonyl-tRNA is produced, which leads to derepression of the threonine biosynthetic operon at high concentrations homoserine (and threonyl-tRNA) the operon is repressed (Savageau, unpublished data).

Recombinant DNA techniques were employed to improve the L-threonine producer. A threonine-deficient mutant of E. coli was transformed by the genes of threonine operon obtained from a-amino-/ -hydroxyvaleric acid (AHV)-resistant and feedback-insensitive mutants to amplify the expression of enzymes and to increase the amount of L-threonine. E. coli mutant strain was also constructed to have amplified genes of threonine operon obtained from AHV-resistant and feedback-insensitive mutant by the action of Mu phage on the chromosomal DNA. This strain is used in France in the practical production of L-threonine. The productivity of bacterial strains developed as the L-threonine producer is summarized in Table 2 [14]. L-Threonine hyperproducing E. coli mutant, which can produce 100 g/1 of L-threonine in 77 h, was constructed by Okamoto et al. who suggested that the strain has some impairment in L-threo-nine uptake function [15]. 

The (I( )-l-amino-2-propanol linker is known to be derived from threonine. In S. enterica, CobD was found to be an enzyme with L-threonine 0-3-phosphate decarboxylase activity, which generates (/f)-l-amino-2-propa-nol phosphate. The enzyme is a pyridoxal phosphate requiring enzyme and the structure of the protein has been determined by X-ray crystallography (Figure 28). The structure of CobD was found to be highly similar to the aspartate aminotransferase family of enzymes. Structures of CobD with substrate and product bound have allowed a detailed mechanism for the enzyme to be proposed, whereby the external aldimine is directed toward decarboxylation rather than aminotransfer. Threonine phosphate, itself, is synthesized from L-threonine by the action of a kinase, which is encoded by pduX The pduX is housed within the propanediol utilization operon rather than the cobalamin biosynthetic operon for reasons that are not clear. 

The tryptophan, histidine, leucine, phenylalanine, and threonine operons are regulated by attenuation. Repressors and activators also act on the promoters of some of these operons, allowing the levels of these amino acids to be very carefully and rapidly regulated. 

The presence of two genes, nisB and nisC, encoding 993- and 414-residue proteins without significant homology to other known proteins, but conserved in several lantibiotic operons, has made them strong candidates for post-trans-lational modifications in the maturation pathway of lantibiotics [40]. Limited similarity between NisB and E. coli IlvA, a threonine dehydratase, was reported and hence a dehydratase function for NisB was suggested [40]. Mutation studies of NisB, NisC, EpiB, EpiC, and SpaB indicated that these proteins were essential for nisin, epidermin and subtilin biosynthesis, respectively [40,86,87,190]. As no precursors have been identified and characterized in these mutants, conclusions about the reaction that is catalyzed by these proteins remain speculative [40]. Secondary-structure predictions and experimental evidence confirmed that NisB and SpaB are both membrane-bound [100]. 

Lee KH, Park JH, Kim TY, Kim HU, Lee SY (2007) Systems metabolic engineering of Escherichia coli for L-threonine production. Mol Syst Biol 3 1-8 Lim SJ, Jung YM, Shin HD, Lee YH (2002) Amplification of the NADPH-related genes zfvf and gnd for the oddball biosynthesis of PHB in an E. coli transformant harboring a cloned phbCAB Operon. J Biosci Bioeng 93 543-549... 

Fig. 1. The L-arabinose gene-enzyme complex showing the L-arabinose operon ara-OIBAD, the L-arabinose regulatory gene araC, and gene araE which is concerned with the active transport of L-arabinose, thr = L-threonine leu = L-leucine thy = thymine ara = L-arabinose.

In other words, isoleucine and valine stabilize the enzyme in the immature form. Their further observation that the immature form of threonine deaminase has an affinity for leucyl tRNA but not for valyl or isoleucyl tRNA is certainly of interest with respect to the obvious correlation with multivalent repression that is possible [16]. Hatfield and Burns propose that the leucyl tRNA-immature tetramer complex might be the holorepressor of the ilv genes or perhaps at least of the ilv ADE operon. If either valine or isoleucine were limiting, maturation of the enzyme would occur without interference by either leucine or leucyl tRNA, thus leading to derepression. Derepression would also occur when isoleucine and valine were both in sufficient excess to block maturation provided leucine were limiting so that the leucyl tRNA-immature tetramer complex could not be formed. The significance of this model has yet to be assessed, but it is difficult at present to account for the supposed involvement of the isoleucyl and valyl tRNA synthetases in multivalent repression. Nevertheless, this interesting observation should be kept in mind as a possible mechanism that could account for part of the phenomenon of multivalent repression when additional information is acquired. 

Zhang X, Yan JA, Yu L, Zhang GQ, Zhang Y, Chen N. Construction of recombinant plasmids containing threonine operon and their effects on L-threonine accumulation. Acta Microbiol Sin 2009 49 591-6. 

L-isoleucine are both in excess, the translation of the short peptide proceeds, resulting in the formation of a terminator structure, so the transcription of the thr operon is pre-terminated. By contrast, when L-threonine and L-isoleucine are lacking, translation of the short peptide stalls, leading to the formation of an antiterminator structure, so the transcription of the thr operon can continue. Mutation of a G insertion at position -37 upstream of thrA could cause derepression, probably through destabilizing the terminator structure (Gardner 1979 Gardner and Reznikoff 1978). 

Furukawa S, OzaM A, Nakanishi T (1988) L-threonine production by L-aspartate- and L-homoseiine-resistant mutant oiEscherichia coli. Appl Microbiol Biotechnol 29 550-553 Gardner JF (1979) Regulation of the threonine operon tandem threonine and isoleucine codons in the control region and translational control of transcription termination. Proc Natl Acad Sci... 

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