6.6.3.2.3 Complex RNA Polymerase II Promoters. A typical example of a eukaryotic pro-
moter that has complex tissue specificity for gene expression occurs in the Drosophilagene, even-skipped
(Figure 6.20). Altogether, 11 regions of the gene control transcriptional initiation in a variety of ways.
Each is associated with a different requirement of the protein product, such as position in the body
(stripes), cell type (neurons, muscle, anal plate ring). Additionally, most of the specificities (shown by the
shading in Figure 6.20) are sub-divided. For example, each of the four neuronal control elements confers
transcriptional induction in a different sub-set of the neurons of the fly.
6.6.3.2.4 Transcriptional Enhancers. The complete region controlling transcriptional initiation
in the even-skippedgene is 9 103 bp long, which demonstrates that transcriptional control elements can
act at great distances. It is thought that the intervening DNA between a distant control element and its
basal transcription machinery is looped out (Figure 6.21). Such distant positive control elements have been
termed enhancersand their negatively regulating analogues are called silencers. Like the other promoter
elements, enhancers and silencers work by binding specific transcription factors, which then interact with
the transcription machinery (Figure 6.21).
6.6.3.2.5 Transcriptional Insulators. An enhancer or silencer can act on more than one transcrip-
tional start site (Figure 6.22). So the cell must be able to prevent enhancers and silencers from working on
228 Chapter 6
Figure 6.19 Assembly of the basal transcriptional initiation complex for RNA polymerase II in eukaryotes.
TF: Transcription factor. TATA, TATA box. TBP, TATA binding protein. Circled P indicates protein
phosphorylation of RNA polymerase II