Stabilization positive feedback

The riocus encodes the enzyme flavonoid 3 -hydroxylase (F3 H) [17, 18], and is an important controller of flux in the anthocyanin pathway in soybean seed coats (Fig. 4.1). F3 H diverts metabolic flux away from biosynthesis of orange (pelargoni-din) and blue (delphinidin) anthocyanins toward the red cyanidin-3-(9-glucoside, which is the main anthocyanin in the seed coats of black soybean [7, 8]. T increases the accumulation of delphidin-3-O-glucoside in black seed coats, even though it is not required for its biosynthesis [19]. Possible mechanisms for this include positive feedback, or the stabilization of the putative anthocyanin biosynthetic metabolon [20] by F3 Fl-derived membrane anchoring (Fig. 4.1). 

Accepting these results for an idealized system confirms the intuitive notion that the use of positive feedback from the current follower is identical to physically moving the reference electrode closer to the working electrode. There are no additional concepts to cope with in relating response to physical parameters. It will become evident that this situation allows a rather simple assessment of loop gain and a clear view of corrective measures that will lead to stability with Ru compensation. 

Hydrodynamic instability, where a small increase in die pressure leads to a larger local residence time, which in turn, through conversion, results in a larger viscosity. This viscosity increase will successively increase the die pressure even further. The positive feedback will be counterbalanced by the back-flow, because an increased viscosity also increases the pressure build-up ability of the extruder. An influence on the stability may be expected if the interaction parameters and the local viscosities are such that the positive feedback dominates. 

Because of the positive feedback effect AHLs have on regulating the transcription of their own synthases, concentrations of AHLs can vary enormously between lower density and higher density cultures, such as biofilms. P. aeruginosa 30Ci2 homoserine lactone (with a keto-group on the third of the 12 carbons of the acyl chain) has been measured to have a concentration of 2-10 tM in a standard lab culture and of up to 600 (xM in the vicinity of an in vitro biofilm (17). However, experimental sensitivity and issues with AHL stability might have influenced the above results. 

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