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504 Gene Regulation in Prokaryotes Just upstream of these (but not shown in the figure) is a CAP site: in the absence of glucose, CAP binds here and helps activation, In the absence of arabinose the araBAD genes are not expressed. This is because, when not bound to arabinose, AraC adopts a different confor- mation and binds DNA in a different way; one monomer still binds the aral, site, but the other monomer binds a distant half-site called araO,, as shown in Figure 16-18b. As these two half-sites are 194 bp apart, when AraC binds in this fashion the DNA between the two sites forms a loop. Also, when bound in this way, there is no monomer of AraC at aral,, and as that is the position from which activation of araBAD promoter is mediated, there is no activation in this configuration. The magnitude of induction of the araBAD promoter by arabinose is very large, and for this reason the promoter is often used in expression vectors. Expression vectors are DNA constructs in which efficient synthesis of any protein can be ensured by fusing its gene to a strong promoter (see Chapter 20). In this case, fusing a gene to the araBAD promoter allows expression of the gene to be controlled by arabinose: the gene can be kept off until expression is desirable, and then “induced” when its product is wanted simply by addition of arabinose. This allows expression even of genes with products that are toxic to the bacterial cells. EXAMPLES OF GENE REGULATION AT STEPS AFTER TRANSCRIPTION INITIATION Amino Acid Biosynthetic Operons Are Controlled by Premature Transcription Termination In E. coli the five contiguous irp genes encode enzymes that syn- thesize the amino acid tryptophan. These genes are expressed effi- ciently only when tryptophan is limiting (Figure 16-19). The genes are controlled by a repressor, just as the Jac genes are, but in this case the ligand that controls the activity of that repressor (tryptophan) acts not as an inducer but as a corepressor. That is, when tryptophan is present, it binds the Trp repressor and induces a conformational change in that protein, enabling it to bind the fp operator and prevent transcription. When the tryptophan concentration is low, the Trp repressor is free of its corepressor and vacates its operator, allowing the synthesis of frp mRNA to commence from the adjacent promoter. : leader trpE trpD trpC trpB trpA operator easy, promoter | PIRES RNA Sy FIGURE 16-19 Theérp operon. The tryptophan operon of E. cali, showing the relation of the leader (see text) to the structural genes that code for the Trp enzymes. The gene products are anthranilate synthetase (product of irpE), phosphoribosyl anthranilate transferase (trp), phosphoribosyl anthranilate isomerase indole glycerol phosphate synthetase (pC), tryptophan synthetase B (tpB), and tryptophan synthetase « (trpA). 
