ABSTRACT
The regulation of transcription in eucaryotes is in general much more complex and currently less well understood than the rather simple switch mechanisms that regulate procaryotic gene expression, examples of which we discussed in Chapter 8. There are three classes of RNA polymerases in eucaryotes. Two of these polymerases, RNA polymerase I and RNA polymerase III, transcribe the genes encoding ribosomal RNAs and transfer RNAs, respectively; genes that code for the messenger RNAs of proteins are transcribed by RNA polymerase II, and it is these genes that will be our principal focus. Complex sets of regulatory elements control the initiation of transcription of these structural genes. Distal to the RNA polymerase II initiation site there are different combinations of specific DNA sequences, each of which is recognized by a corresponding site-specific DNA-binding protein. These proteins are called transcription factors, and each combination of DNA sequence and cognate transcription factor constitutes a control module. The essence of transcriptional regulation in eucaryotes is to use different combinations of a large set of control modules to regulate the expression of each gene. Given the large number of modules that is now being discovered in the human genome, the number of possible combinations is almost unlimited.
