The co-ordinated expression of a gene involves DNA elements in the vicinity of the gene and many protein factors. The level of complexity is somewhat different between prokaryotes and eukaryotes. In bacteria, all the DNA sequences required for correct control of transcription are located within a few hundred base pairs of the transcription start (see Figure 8.1). Immediately upstream from the transcription start is the RNA polymerase binding site and then upstream from this there may be activator binding sites. Repressor binding sites may be located at different positions within the promoter; they may overlap, be upstream of or downstream of the RNA polymerase binding site. Figure 11.1a shows the sequence elements required for the regulation of the lac operon from E. coli.