ABSTRACT
Simple DNA sequence (where sequences are represented in the chromosome once or only a few times), multiple structural genes often existing within one transcription unit, very rapid turnover of messenger RNA and concurrent transcription and translation limit the scope for regulation to occur at any point other than at the level of transcription in prokaryotes. This chapter concerns one stage in the overall process of expression of genetic information in eukaryotic cells: the role of DNA-dependent RNA polymerases in the controlled expression of specific genes. The common approach to the isolation of RNA polymerases is to resolve nuclei from whole cells or tissues. The absolute numbers of enzyme molecules present in a system may be defined using specific antisera raised to specific RNA polymerase species, bearing in mind the known sharing of some antigenic determinants between polymerases.
