Endproduct repression

Another control over termination that has been extensively studied is the attenuation of transcription at the end of the leader regions of several amino acid biosynthetic operons (or genes) in E. coVl and related organisms (39). The primary indication that such a mechanism is involved in the control of an amino acid biosynthetic pathway is an endproduct repression that is dependent on the amino acid being transferred to its cognate tRNA at an ample rate. Thus, derepression of the histidine biosynthetic pathway can be achieved by any mechanism that reduces the intracellular level of histidyl tRNA, for example, by limiting the supply of histidine itself or by limiting the activity of the histidyl tRNA synthetase (51). 

Fig. 157. Fine regulation by isoenzymes biosynthesis of the aspartate family of amino acids. ER = endproduct repression, EH = endproduct inhibition. The synthesis of methionine is also subject to-feedback mechanisms, which are, however, now shown here (cf. also Fig. 156).

A feedback inhibition has been detected in B. subtilis, using the ferrisiderophore reductase. This enzyme reduces iron from the ferrisiderophore. The rate at which the ferrisiderophore reductase reduces iron from ferrisiderophores may signal the aromatic pathway about the demand for chorismic acid for 2,3-DHBA synthesis [128,129]. The reductase may have a regulatory effect on chorismate synthase activity. Chorismate synthase may have oxidizable sulfhydryl groups that, when oxidized, may slow the synthesis of chorismic acid [128-130]. There seemed to be no repression or inhibitory effect of 2,3-DHBA or SA on its own biosynthesis [78,121]. Also the endproduct mycobactin (sole endproduct) does not inhibit SA biosynthesis [78]. 

Just as physiological patterns of control over metabolite flow can be recognized, so can physiological patterns of control over the amount of enzyme. Table III lists a convenient classification. Clearly, the amount of enzyme at any time is a summation of the amount formed and the amount destroyed. Two specific patterns controlling formation might be recognized repression by endproducts and induction by substrate or a precursor. It should be stressed that the terms "repression" and "induction" are operational and should carry no connotations regarding the mechanisms. 

Figure 2, The biosynthesis of lysine, methionine, threonine, and isoleucine in E. coli and S. marcescens. Solid arrows, steps catalyzed by enzymes repressed by lysine. Broken arrows, steps catalyzed by enzymes repressed by methionine. Open arrows, steps catalyzed by enzymes repressed by threonine plus isoleucine. Open, dashed arrows, steps catalyzed by enzymes controlled as described in Figure 1. Structural genes indicated in italics. Dashed lines indicate reactions controlled by endproduct inhibition. Reproduced, with permission, from Ref. 57. Copyright 1975, American...

In an inducible enzyme system, the R. is inactive in the presence of the effector (inducer) binding to the inducer apparently changes the conformation of the R., so that it no longer binds to the operator (see Enzyme induction). Synthesis of mRNA can therefore proceed only when the inducer is present. In enzyme repression, the situation is revets R. is activated by a corepressor (the endproduct of a biosynthetic pathway, e.g. an amino acid) so that it can bind to the operator. In this case, synthesis of mRNA proceeds only in the absence of corepressor (see Enzyme repression). See also Derepression. 

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