Process control, correlation regulators

For years, it was assumed that the rate of RNA synthesis was the rate-limiting step that controls indirectly the amount of protein synthesized. However, this simplistic view has been replaced by results clearly showing that mRNA and protein levels do not always correlate fully, and that extensive regulation of mRNA transcript processing and availability for translation occurs. 

A similar approach was used to discriminate between intrinsic noise, stochastic fluctuations that arise from fluctuations of the molecules directly involved in gene expression, and extrinsic contributions that arise form fluctuations in other molecules that regulate or otherwise contribute to the process [22]. The latter are uniform within a cell but vary from cell to cell while the former produce fluctuations over time within an individual cell. The two forms of noise were distinguished by determining the extent of correlation between the expression of cyan and yellow fluorescent proteins under the control of identical promoters in the same cell. The intrinsic noise was measured as the difference in fluorescence intensity of the YFP and CFP. The extrinsic noise was determined by using the fact that the square of the total noise equals the sum of the squares of the intrinsic and extrinsic noises. The results show that both types of noise contribute significantly. 

Enzyme Changes in Zinc Deficiency. Since zinc is required for many enzymes, it is reasonable to speculate that the level of zinc in cells controls the physiological processes through the formation and/or regulation of activity of zinc-dependent enzymes. Until 1965, there was no evidence in the literature to support this concept. During the past decade it has been shown that the activity of various zinc-dependent enzymes was reduced in the testes, bones, esophagus, and kidneys of zinc-deficient rats in comparison with their pair-fed controls (90,91,100,101), These results correlated with the decreased zinc content in the above tissues... 

---