The same principles of transcriptional regulation apply to both prokaryotes and eukaryotes. It is assumed that the concepts in the previous chapter have been understood before continuing into eukaryotic transcriptional regulation. However, the regulation of transcription in eukaryotes, especially multicellular organisms, is more complex than that in prokaryotes. Higher eukaryotes have many more genes than bacteria and regulate their expression differently in different tissues of the body and at different stages of development. For transcription to occur, the DNA must first be exposed. In general, expression of a eukaryotic gene requires the presence of several activators. These may bind to the upstream region of the promoter or to enhancer sequences that may be several kilobases away from the promoter, as described briefly in Ch. 6. Furthermore, eukaryotic enhancers may lie downstream of their target genes and work in either orientation.
Eukaryotic genes are sequestered in the nucleus. Since transcription factors are proteins, they are made by ribosomes in the cytoplasm, but to act they must enter the nucleus. Although both bacterial and eukaryotic DNA are condensed and covered with protein this is much more pronounced in eukaryotic cells. Here the DNA highly condensed into nucleosomes and covered with histones. Long sections of the DNA are frequently folded tightly into heterochromatin and are therefore inaccessible to RNA polymerase (Fig. 10.01). This makes access to the DNA difficult, both for RNA poly-merase and transcription factors.
FIGURE 10.01 Eukaryotic Genes Are Difficult to Access
Compartmentalization of the eukaryotic cell into nucleus and cytoplasm means that the transcription factors must be made in the cytoplasm and transported to the nucleus. The DNA in the nucleus is often highly condensed and difficult to access.
Protein coding genes of eukaryotes are regulated by transcription factors that respond to specific signals.
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