Repressor Eucaryotic

The specific arrangement of two a helices joined by a loop region in lambda Cro and repressor, as well as in CAP, constitute the helix-turn-helix DNA-binding motif (Figure 8.8), which also occurs in some eucaryotic transcription factors as discussed in Chapter 9. The orientation of the two helices and... 

The DNA-binding motifs discussed in this and the preceding two chapters are those most frequently found in procaryotes and eucaryotes. However, other motifs are known, for example the p sheet motif of the met repressor in Escherichia coli which binds to the major groove of DNA. No doubt others remain to be discovered. 

P-sheet structures as DNA-binding motifs are found in pro- and eucaryotic DNA-binding proteins. As an example, the structure of the MetJ repressor from E. coli is shown in Fig. 1.9. The DNA is contacted in the major groove by the protruding P-strands. 

As in procaryotes, there are transcription activating proteins, the transcriptional activators, as well as proteins which inhibit transcription, the transcriptional repressors. Of the two classes, the activators are the most extensively studied and characterized. The mechanism of eucaryotic repressors has only been clarified in a few cases. 

A further significant mechanism of transcription control is the repression of gene expression (review Cowell, 1994 Johnson, 1995). There are two types of gene repression to be distinguished in eucaryotes. On the one hand the chromatin structure can cause an unspecific repression of gene expression (see 1.4.6). On the other hand, analogous to the specific transcriptional activators, there are specific repressors of transcription. Their effect, in contrast to that of vmspecific repressors, is sequence-dependent and thus suitable for selective repression (Fig. 1.40). DNA sequences that mediate repression of transcription factors are termed silencers. 

The helix-tum-helix motif (HTH motif) is - historically seen - the first DNA-binding motif whose structure could be solved in a complex with DNA. It is often found in bacterial repressors. Many eucaryotic DNA-binding proteins also utilize the helix-turn-... 

The formation of an active, regulation-competent initiation complex for transcription in eucaryotes demands the concerted action of a large number of proteins. It is estimated that more than 100 different proteins participate in the initiation of transcription in eucaryotes. The basal transcription complex, consisting of the general initiation factors, as well as RNA polymerase II, only allows for a slow transcription rate. For a regulated acceleration of this low transcription rate by transcriptional activators, further proteins named coactivators or corepressors are required. The terms coactivator or corepressor are miscellaneous terms referring to proteins that are required for the functions of transcriptional activators (or repressors) but are not required for... 

Lin, S., and Riggs, A. D., 1975a, The general affinity of lac repressor for E. coli DNA Implications for gene regulation in procaryotes and eucaryotes. Cell 4 107. 

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