Isoleucine operon

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. 

It is proposed that the ilv U gene is an isoleucine-specific regulatory gene that controls the level of isoleucyl tRNA synthetase activity. Only when the activating enzyme is quite low does the high level of derepression of the ilv ADE operon occur. It does appear that both the ilv U and ilv T lesions by themselves have some direct effect on derepression of this same operon and resistance to thiaisoleucine. 

Because of the superficial similarity of the azaleucine-resistant (azlO E. coli strains that had derepressed levels of all four leucine and four of the five isoleucine-valine biosynthetic enzymes to the leu S mutants of S. typhimurium, they have been examined in greater detail than the other azl" strains. However, examination has shown that these mutants have normal levels of leucyl tRNA synthetase [2-7]. In view of the fact that some of the lesions leading to derepression of the his operon appear to affect the levels of IRNA , the kind and amounts of tRNA species... 

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. 

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... 

The two well-studied lipopeptide bioemulsifiers produced by Bacilli—surfactin (Peypoux et al. 1999) and lichenysin (Yakimov et al. 1995)—are structurally similar. As already mentioned, both are composed of a cyclic heptapeptide linked to a fatty acid and differ in the last amino acid of the peptide—leucine and isoleucine, respectively. The peptide moiety, like many small peptides in microorganisms, is synthesized non-ribosomally by a multienzyme peptide synthetase complex (Marahiel 1997). The srfA operon of B. subtilis was defined by a transposon... 

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