Positive regulatory mechanism

The enzymes that convert IMP to XMP and adenylosuccinate are both regulated. GMP inhibits the activity of IMP dehydrogenase, and AMP inhibits adenylosuccinate synthetase. Note that the synthesis of AMP is dependent on GTP (of which GMP is a precursor), whereas the synthesis of GMP is dependent on ATP (which is made from AMP). This serves as a type of positive regulatory mechanism to balance the pools of these precursors when the levels of ATP are high, GMP will be... 

Fig. 7. Sequential control of the synthesis of the aspartate family of amino adds. Temporal control of the flow of carbon is illustrated by the successive Figs. 1-5. Potential quantitative changes in flow are approximated by the thickness of the solid arrows. As the concentration of an end product is increased (indicated by closed boxes), the pattern of synthesis is altered by utilization of negative (-) or positive (+) regulatory mechanisms as described in the text. The pattern of control which is illustrated assumes that aspartate kinase is sensitive to inhibition only by lysine. Variations of this pattern are discussed in the text.

A rather satisfactory explanation of the irreversibility of amino acid accumulation in yeast cells is that it might result from specific regulatory mechanisms capable of immobilizing the transporters in a closed position. Uptake of amino acids by a number of permeases does indeed appear to be regulated by specific, and possibly allosteric, feedback inhibition. This idea is based on the fact that a number of transport systems seem to be specifically inhibited by their internally accumulated... 

Additional regulatory mechanisms can involve various host proteins (i.e., not encoded by the transposon) that are involved in transposome assembly and/or activity. For example, the Escherichia coli histone-like proteins IHF and HU are required for bacteriophage Mu. IHF and HNS, although not required, stimulate TnlO (IS70) transposition. A more systematic smdy has revealed several additional host factors that affect transposition of various TE in E. coli either positively or negatively (15). Finally, in the case of the eukaryotic transposon. Sleeping Beauty, the HMG protein is required for integration. 

The formation and maintenance of the myelin sheath require the coorcUnafion of a number of gene products. While some gene products facilitate myelination, some others try to suppress myelin formation. In the following lines, we describe such positive and negative regulatory mechanisms. 

In addition, we have used the positions and orientations of fc-mers within their respective conserved sets to show that many of these -mers have position and orientation biases (1,3). This provides both additional evidence for functionality of these predicted regulatory elements, and additional insights into the underlying regulatory mechanisms, possibly leading to more focused validation experiments. 

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