Transcriptional units

The multiple sites that serve as origins for DNA replication in eukaryotes are poorly defined except in a few animal viruses and in yeast. However, it is clear that initiation is regulated both spatially and temporaUy, since clusters of adjacent sites initiate rephcation synchronously. There are suggestions that functional domains of chromatin replicate as intact units, implying that the origins of rephcation are specificaUy located with respect to transcription units. 

A third class of sequence elements can either increase or decrease the rate of transcription initiation of eukaryotic genes. These elements are called either enhancers or repressors (or silencers), depending on which effect they have. They have been found in a variety of locations both upstream and downstream of the transcription start site and even within the transcribed portions of some genes. In contrast to proximal and upstream promoter elements, enhancers and silencers can exert their effects when located hundreds or even thousands of bases away from transcription units located on the same chromosome. Surprisingly, enhancers and silencers can function in an orientation-independent fashion. Literally hundreds of these elements have been described. In some cases, the sequence requirements for binding are rigidly constrained in others, considerable sequence variation is... 

Figure 43-11. The hormone response transcription unit. The hormone response transcription unit is an assembly of DNA elements and bound proteins that interact, through protein-protein interactions, with a number of coactivator or corepressor molecules. An essential component is the hormone response element which binds the ligand (A)-bound receptor (R). Also Important are the accessory factor elements (AFEs) with bound transcription factors. More than two dozen of these accessory factors (AFs), which are often members of the nuclear receptor superfamily, have been linked to hormone effects on transcription. The AFs can interact with each other, with the liganded nuclear receptors, or with coregulators. These components communicate with the basal transcription complex through a coregulator complex that can consist of one or more members of the pi 60, corepressor, mediator-related, or CBP/p300 families (see Table 43-6).

The molecular analysis of the Sh K channel transcription unit indicated that at least ten different channel subunits may be expressed in Drosophila cells. This observation raised a number of important questions only some of them can be addressed here. Each subunit has been expressed in the Xenopus oocyte expression system [11,12,29,39]. These experiments indicate that each subunit expresses distinct... 

B34. Brady, H. J. M., Sowden, J. C Edwards, M., Lowe, N., and Butterworth, P. H. W Multiple GF-1 binding sites flank the erythroid specific transcription unit of the human carbonic anhydrase I gene. FEBS Lett. 257,451-456 (1989). 

Ripoche MA, Kress C, Poirier F, Dandolo L 1997 Deletion of the HI 9 transcription unit reveals the existence of a putative imprinting control element. Genes Dev 11 1596-1604... 

Stromberg P, Hoog JO. Human class V alcohol dehydrogenase (ADH5) a complex transcription unit generates C-termi-nal multiplicity. Biochem Biophys Res Commun 2000 278 544-549. 

Terminator A DNA sequence sited at the end of a transcriptional unit that signals the end of transcription. [NIH]... 

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