Catabolite regulatory protein

In prokaryotic cells, cAMP binds to catabolite regulatory protein (CAP), which then binds to DNA and affects gene expression (Chapter 26). In eukaryotic cells, cAMP binds to cAMP-dependent protein kinase, which contains two regulatory (R) and two catalytic (C) subunits. Upon binding of cAMP, the catalytic subunits separate, become active. 

Regulatory proteins Insulin Somatotropin Thyrotropin lac repressor NEl (nuclear factor 1) Catabolite activator protein (CAP) API... 

The organism senses whether glucose is available by another regulatory mechanism which cooperates with the lac repressor and the lac operator. The promoter is therefore sub-divided into two specific regions, each of distinctive function. One is the RNA polymerase entry site, where RNA polymerase first becomes bound to DNA (cf. DNA transcription, Appendix 5.6), and the other is the protein binding site for the catabolite activator protein (CAP) (Fig. 5.39). The CAP protein binding site controls the polymerase site which, when bound to the DNA, allows successful transcription provided that the repressor is not bound. When the CAP protein is not bound, then RNA polymerase cannot bind and transcription cannot take place. 

Umland, T. C., Taylor, K. L., Rhee, S., Wickner, R. B., and Davies, D. R. (2001). The crystal structure of the nitrogen catabolite regulatory fragment of the yeast prion protein Ure2p. Proc. Natl. Acad. Sci. USA, in press. 

The expression of patA gene is under the control of NtrC (nitrogen regulatory protein C) and (Zimmer et al. 2000 Samsonova et al. 2003 Schneider et al. 2013) and is also subjected to catabolite repression (Shaibe et al. 1985b). Because loss of both o and diminished PatA activity, patA is transcribed with RNA polymerase not only with but also with o (Schneider et al. 2013). The expression of ydcSTUVW (ydcW=patD) operon is regnlated exceptionally by Nac (nitrogen assimilation control protein) and o (Schneider et al. 2013). 

Glucose is the preferred carbon source for yeast, as it is for bacteria. When glucose is present, most of the GAL genes are repressed—whether galactose is present or not. The GAL regulatory system described above is effectively overridden by a complex catabolite repression system that includes several proteins (not depicted in Fig. 28-29). 

Fig. 16.7. Catabolite repression of stimulatory proteins. The lac operon is used as an example. A. The inducer allolactose (a metabolite of lactose) inactivates the repressor. However, because of the absence of the required coactivator, cAMP-CRP, no transcription occurs unless glucose is absent. B. In the absence of glucose, cAMP levels rise. cAMP forms a complex with the cAMP receptor protein (CRP). The binding of the cAMP-CRP complex to a regulatory region of the operon permits the binding of RNA polymerase to the promoter. Now the operon is transcribed, and the proteins are produced.

In the foregoing presentation, two regulatory mechanisms were considered both of which might produce the same pattern of exoprotein secretion, that is, a low rate during the exponential phase and a much higher rate afterwards. It is suggested that where exoprotein formation represents a minor fraction of total bacterial protein production catabolite repression may be important whereas in cases where it accounts for a major portion "competition" may be of greater importance. This hypothesis can be examined under the test conditions described by Mandelstam (1961, 1962). 

The results cited above indicate that there is no regulation of the synthesis of membrane proteins by the composition of the cell membrane. We know that the synthesis of some membrane proteins, such as the lac-transport protein in E. coli, is controlled at the level of transcription by the normal regulatory processes of the lac operon. The same type of control may apply for other membrane proteins as well. In other words, their synthesis is controlled by regulatory processes that are independent of the state of the membrane, but dependent upon other physiological parameters of the cell, such as positive and negative transcriptional controls, catabolite repression, the general level of energy metabolism and the availability of compounds necessary for protein synthesis. 

---