Regulation by External Factors

The procedures for studying mechanisms of regulation of the rates of biochemical processes are described in Chapter 3. These procednres are applied to peptide synthesis in this discussion. There are three qnestions that must be answered prior to discussion of mechanisms, (i) Which reactions are non-eqnilibrium (ii) Which is the flnx-gen-erating reaction and how is it regulated (iii) What reactions other than the flnx-generating steps are regulated by external factors, i.e. external to the intermediates in peptide synthesis ... 

Fair-skiimed humans face a dilemma because ultraviolet light exposure carries the risks of carcinogenesis and skin aging, whereas insufficient ultraviolet light carries the risk of vitamin Ds (94, Fig. 11) insufficiency. The case is unique in so far as an endogenous biosynthetic pathway is subject to regulation by external lifestyle factors such as ultraviolet exposure and the use of chemical and physical sunscreens. Recent studies... 

In a mammalian cell, a large number (about 10 000) of mRNAs have a low abundance, i.e., a few copies per cell. Many strategies have been designed to enrich for specific mRNAs or to recognize certain clones (Section 2.5.2.2) otherwise many clones would have to be analyzed (Table 10.1). Subtractive hybridization depletes the sample of nonspecific mRNA in order to enrich for the mRNA sought. It is particularly suitable for isolating tissue-specific or developmental regulated sequences or clones derived from mRNA induced by external factors. 

Up to now we have spoken only of regulation by internal factors. However, the situation within an organism is influenced quite substantially by external factors. Of the many external factors which can influence the growth and differentiation of plants, temperature and light are selected here. Moreover, we can point out only a few of the fundamentals relating to the mechanism of action of temperature and light in this section. 

Transcriptional regulation encompasses the modulation of the synthesis rate of mRNA due to cell type specific factors that may be triggered by external stimuli. 

In undisturbed ecosystems, plant and herbivorous Insect populations coexist In a steady state condition dictated by external biological and physical factors (parasites, predators, precipitation, temperature, soil quality, etc.). But this balance is also regulated by myriads of very fundamental Interactions, many or even most of which are yet unknown, between the plant allelochemi-cals and the biochemical, physiological, and behavioral functions of the Insect herbivores ( 1). 

The discovery of the effects of gibberellin on flower formation is closely linked 1 with older studies on the physiology of this process, particularly its regulation by certain external or environmental factors. 

The regulated expression of inducible promoters offers advantages for selective activation of promoter activity in desired tissues. Activation can be achieved by either external factors (ionizing or non-ionizing radiations in the case of heat shock or radiation-inducible promoters), physiological characteristics unique to certain disease states, such as hypoxia in cancer, or the administration of non-toxic agents, such as glucose. 

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