Intracellular Regulatory Mechanisms: Regulation in multicellular forms may be an elaboration upon the pattern evolved in microorganisms.
The study of metabolic regulation in microorganisms has revealed several simple but efficient regulatory circuits. In one, the operation of an entire sequence of enzymes is controlled by the activity of the initial enzyme which contains a specific inhibitor site. When this site is combined with the endproduct of the sequence, the catalytic site is rendered inactive. In another, the formation of an entire sequence of enzymes is controlled by means of a cytoplasmic mediator which blocks the transcription of the genetic message (repression) when activated by the endproduct, or which allows the transcription (induction) when activated by the substrate of the first enzyme in the sequence. Additional circuits have been proposed for the regulation of RNA and DNA synthesis. The same regulatory devices could account, in part, for intracellular metabolic control in more complex animal and plant forms. However, superimposed upon these simple control circuits will be found others which take advantage of the greater degree of organization in these cells and of the possibilities for regulating gene function that are provided by the chromosomes. The pattern of proteins with special control sites, such as have evolved in the relatively simple controls found in bacteria, may also be found essential for intercellular controls involving nervous and humoral mechanisms.
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