6.6.2.2.4 Regulatory Proteins Affecting Transcriptional Initiation in E. coli. Promoters do
not necessarily have the same activity under all conditions. Some are induced and/or repressed under dif-
ferent conditions, such as the need or not for the protein product. These activities are mediated by regula-
tory proteins that bind to the promoter region. In the bacterium E. coli, paradigms for such inducers and
repressors have been studied in great detail. The lac operon, which encodes the enzymes for metabolising
lactose, shows both induction and repression phenomena (Figure 6.18).3,35
E. colicell uses lactose as a source of sugar but the operon is switched off in its absence. This is
achieved by the lacrepressor, which, in the absence of lactose, binds to a control region (the lacoperator),
downstream of the lac operon promoter, to shut off transcriptional initiation. However if lactose is present
in the cell, one of its metabolites, allolactose, binds to the lac repressor and blocks its ability to bind to the
operator. Furthermore, in cells containing both lactose and glucose, the lacoperon is virtually inactive
(glucose is a more attractive source of energy than lactose). This effect is mediated by the catabolite acti-
vator protein(CAP),^35 which activate the promoters of several genes, which encode enzymes that
metabolise sugars other than glucose. A cell containing glucose has low cyclic AMP (cAMP) levels, lead-
ing to a loss of the ability of CAP to bind to its target. This leads to an almost complete switch off of all
these operons. If glucose falls below a threshold level, cAMP levels rise, the CAP protein attaches to its
binding site, and the operons are induced.
6.6.2.2.5 Promoter Specificity in E. coliis Regulated by Different Factors. Most E. coligenes
are transcribed with the aid of a single factor (70) but other genes need to be turned on under specific cir-
cumstances. For example, heat shock or nitrogen starvation induces the transcription of a series of genes that
have different promoters. Such promoters have variant35 boxes and ‘Pribnow-like’ boxes (Table 6.2).^33
6.6.3 Transcription in Eukaryotes
6.6.3. 1Eukaryotic RNA Polymerases. Eukaryotes have three different nuclear RNA polymerases,
RNA polymerases I, II and III, as well as separate enzymes for their chloroplasts and/or mitochondria.
RNA polymerase I transcribes rRNA exclusively.^36 RNA polymerase IItranscribes all protein-coding
226 Chapter 6
Table 6.2 Different sfactors in E. coli
factor Use 35 sequence Gap (base pairs) 10 sequence
70 General TTGACA 16–18 TATAAT
32 Heat shock CNCTTGAA 13–15 CCCCATNT
54 Nitrogen starvation CTGGNA 6 TTGCA
Figure 6.18 Activators and repressors of transcriptional initiation in E. coli – the lac operon. Activation and repres-
sion of transcription are indicated by () and () respectively