Gene expression transcriptional state

Clearly, the control of gene expression at the transcriptional level is a key regulatory mechanism controlling carotenogenesis in vivo. However, post-transcriptional regulation of carotenoid biosynthesis enzymes has been found in chromoplasts of the daffodil. The enzymes phytoene synthase (PSY) and phytoene desaturase (PDS) are inactive in the soluble fraction of the plastid, but are active when membrane-bound (Al-Babili et al, 1996 Schledz et al, 1996). The presence of inactive proteins indicates that a post-translational regulation mechanism is present and is linked to the redox state of the membrane-bound electron acceptors. In addition, substrate specificity of the P- and e-lycopene cyclases may control the proportions of the p, P and P, e carotenoids in plants (Cunningham et al, 1996). 

The details of the mechanism by which CREB influences gene expression are becoming increasingly understood (Fig. 23-9) [62, 63]. In the basal, or unstimulated, state, CREB is bound to its CREs but does not alter transcriptional rates under most circumstances. Stimulation of a cell by a variety of first messengers leads to the phosphorylation and activation of CREB, which then leads to the regulation of gene transcription. Such phosphorylation of CREB occurs on a single serine residue, serine 133, and can be mediated by one of several protein kinases. 

The process of transcription runs under quite complicated control mechanisms. Among other things, these mechanisms direct the RNA synthesis to start and stop in precise places on the DNA, and they control the rate of mRNA synthesis. In this way gene expression can be integrated with the metabolic state of the cell, and with the cell cycle for cellular replication and differentiation. 

E. coli maintains all of its genes in a state where they can be turned on or turned off on short notice. The short messenger lifetime makes it possible to control gene expression from the transcription level. The lack of separate compartments for RNA and protein synthesis has fostered mechanisms where translation actually exerts a direct role on transcription. These are some of the special features that have influenced the evolution of regulatory systems in E. coli. 

Attempts to understand the molecular basis for the induction of synthesis of specific proteins/enzymes are discussed in most of the chapters, but some topics are more advanced in this respect than others. In most cases induction is due to activation of gene expression, as measured by increased steady state levels of mRNA, but in some cases, for instance nitrate reductase, post-transcriptional events may also be important. The eventual aim of course is to understand the series of events - the so-called signal transduction pathway - which lead from perception of the signal to increased mRNA levels and hence increased enzyme synthesis. In a... 

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