Valine operon

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. 

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