Regulation of Gene Expression in Prokaryotes Art and Photos in PowerPoint® Concepts of Genetics Ninth Edition Klug, Cummings, Spencer, Palladino Chapter 17 Regulation of Gene Expression in Prokaryotes Copyright © 2009 Pearson Education, Inc.

17.1 Prokaryotes Regulate Gene Expressions in Response to Environmental Conditions

17.2 Lactose Metabolism in E. coli Is Regulated by an Inducible System

Figure 17-1 A simplified overview of the genes and regulatory units involved in the control of lactose metabolism. (This region of DNA is not drawn to scale.) A more detailed model will be developed later in this chapter. (See Figure 17–10.) Figure 17.1

17.2 Lactose Metabolism in E. coli Is Regulated by an Inducible System 17.2.1 Structural Genes

Figure 17-2 The catabolic conversion of the disaccharide lactose into its monosaccharide units, galactose and glucose. Figure 17.2

Figure 17-3 The structural genes of the lac operon are transcribed into a single polycistronic mRNA, which is translated simultaneously by several ribosomes into the three enzymes encoded by the operon. Figure 17.3

Figure 17-4 The gratuitous inducer isopropylthiogalactoside (IPTG).

17.2 Lactose Metabolism in E. coli Is Regulated by an Inducible System 17.2.2 The Discovery of Regulatory Mutations

17.2 Lactose Metabolism in E. coli Is Regulated by an Inducible System 17.2.3 The Operon Model: Negative Control

Figure 17-5 The components of the wild-type lac operon and the response in the absence and the presence of lactose. Figure 17.5

Figure 17-5a The components of the wild-type lac operon and the response in the absence and the presence of lactose. Figure 17.5a

Figure 17-5b The components of the wild-type lac operon and the response in the absence and the presence of lactose. Figure 17.5b

Figure 17-5c The components of the wild-type lac operon and the response in the absence and the presence of lactose. Figure 17.5c

Figure 17-6a The response of the lac operon in the absence of lactose when a cell bears either the or the mutation. Figure 17.6a

Figure 17-6b The response of the lac operon in the absence of lactose when a cell bears either the or the mutation. Figure 17.6b

17.2 Lactose Metabolism in E. coli Is Regulated by an Inducible System 17.2.4 Genetic Proof of the Operon Model

constitutive trans element cis element Table 17-1 A Comparison of Gene Activity Table 17.1

Figure 17-7 The response of the lac operon in the presence of lactose in a cell bearing the IS mutation. Figure 17.7

17.3 The Catabolite-Activating Protein (CAP) Exerts Positive Control over the lac Operon

Figure 17-8 Catabolite repression Figure 17-8 Catabolite repression. (a) In the absence of glucose, cAMP levels increase, resulting in the formation of a CAP–cAMP complex, which binds to the CAP site of the promoter, stimulating transcription. (b) In the presence of glucose, cAMP levels decrease, CAP–cAMP complexes are not formed, and transcription is not stimulated. Figure 17.8

Figure 17-8a Catabolite repression Figure 17-8a Catabolite repression. (a) In the absence of glucose, cAMP levels increase, resulting in the formation of a CAP–cAMP complex, which binds to the CAP site of the promoter, stimulating transcription. (b) In the presence of glucose, cAMP levels decrease, CAP–cAMP complexes are not formed, and transcription is not stimulated. Figure 17.8a

Figure 17-8b Catabolite repression Figure 17-8b Catabolite repression. (a) In the absence of glucose, cAMP levels increase, resulting in the formation of a CAP–cAMP complex, which binds to the CAP site of the promoter, stimulating transcription. (b) In the presence of glucose, cAMP levels decrease, CAP–cAMP complexes are not formed, and transcription is not stimulated. Figure 17.8b

Figure 17-9 The formation of cAMP from ATP, catalyzed by adenyl cyclase.

17.4 Crystal Structure Analysis of Repressor Complexes Has Confirmed the Operon Model

Figure 17-10 A detailed depiction of various regulatory regions involved in the control of genetic expression of the lac operon, as described in the text. The numbers on the bottom scale represent nucleotide sites upstream and downstream from the initiation of transcription. Figure 17.10

Figure 17-11c Models of the lac repressor and its binding to operator sites in DNA, as generated from crystal structure analysis. (a) The repressor monomer. The arrow points to the inducer-binding site. The DNA-binding region is shown in red. (b) The repressor dimer bound to two 21-base-pair segments of operator DNA (shown in blue). (c) The repressor and CAP (shown in dark blue) bound to the lac DNA. Binding to operator regions O1 and O3 creates a 93-base-pair repression loop of promoter DNA. Figure 17.11c

17. 5. The Tryptophan (trp) Operon in E 17.5 The Tryptophan (trp) Operon in E. coli Is a Repressible Gene System

Figure 17-12 (a) The components involved in the regulation of the tryptophan operon. (b) Regulatory conditions are depicted that involve either activation or (c) repression of the structural genes. In the absence of tryptophan, an inactive repressor is made that cannot bind to the operator (O), thus allowing transcription to proceed. In the presence of tryptophan, it binds to the repressor, causing an allosteric transition to occur. This complex binds to the operator region, leading to repression of the operon. Figure 17.12

Figure 17-12a (a) The components involved in the regulation of the tryptophan operon. (b) Regulatory conditions are depicted that involve either activation or (c) repression of the structural genes. In the absence of tryptophan, an inactive repressor is made that cannot bind to the operator (O), thus allowing transcription to proceed. In the presence of tryptophan, it binds to the repressor, causing an allosteric transition to occur. This complex binds to the operator region, leading to repression of the operon. Figure 17.12a

Figure 17-12b (a) The components involved in the regulation of the tryptophan operon. (b) Regulatory conditions are depicted that involve either activation or (c) repression of the structural genes. In the absence of tryptophan, an inactive repressor is made that cannot bind to the operator (O), thus allowing transcription to proceed. In the presence of tryptophan, it binds to the repressor, causing an allosteric transition to occur. This complex binds to the operator region, leading to repression of the operon. Figure 17.12b

Figure 17-12c (a) The components involved in the regulation of the tryptophan operon. (b) Regulatory conditions are depicted that involve either activation or (c) repression of the structural genes. In the absence of tryptophan, an inactive repressor is made that cannot bind to the operator (O), thus allowing transcription to proceed. In the presence of tryptophan, it binds to the repressor, causing an allosteric transition to occur. This complex binds to the operator region, leading to repression of the operon. Figure 17.12c

17.6 Attenuation Is a Critical Process during the Regulation of the trp Operon in E. coli

Figure 17-13 Diagram of the involvement of the leader sequence of the mRNA transcript of the trp operon of E. coli during attenuation. (a) The -leader sequence of the trp operon is expanded to identify the portions encoding the leader peptide (LP) and the attenuator region (A). Critical nucleotide sequences, including the UGG triplets that encode tryptophan, are also identified; (b) the terminator hairpin, which forms when the ribosome proceeds during translation of the trp codons; and (c) the antiterminator hairpin, which forms when the ribosome stalls at the trp codons because tryptophan is scarce. Figure 17.13

Figure 17-13ab Diagram of the involvement of the leader sequence of the mRNA transcript of the trp operon of E. coli during attenuation. (a) The -leader sequence of the trp operon is expanded to identify the portions encoding the leader peptide (LP) and the attenuator region (A). Critical nucleotide sequences, including the UGG triplets that encode tryptophan, are also identified; (b) the terminator hairpin, which forms when the ribosome proceeds during translation of the trp codons; and (c) the antiterminator hairpin, which forms when the ribosome stalls at the trp codons because tryptophan is scarce. Figure 17.13ab

Figure 17-13c Diagram of the involvement of the leader sequence of the mRNA transcript of the trp operon of E. coli during attenuation. (a) The -leader sequence of the trp operon is expanded to identify the portions encoding the leader peptide (LP) and the attenuator region (A). Critical nucleotide sequences, including the UGG triplets that encode tryptophan, are also identified; (b) the terminator hairpin, which forms when the ribosome proceeds during translation of the trp codons; and (c) the antiterminator hairpin, which forms when the ribosome stalls at the trp codons because tryptophan is scarce. Figure 17.13c

Figure 17-13d Diagram of the involvement of the leader sequence of the mRNA transcript of the trp operon of E. coli during attenuation. (a) The -leader sequence of the trp operon is expanded to identify the portions encoding the leader peptide (LP) and the attenuator region (A). Critical nucleotide sequences, including the UGG triplets that encode tryptophan, are also identified; (b) the terminator hairpin, which forms when the ribosome proceeds during translation of the trp codons; and (c) the antiterminator hairpin, which forms when the ribosome stalls at the trp codons because tryptophan is scarce. Figure 17.13d

17.7 TRAP and AT Proteins Govern Attenuation in B. subtilis TRAP = trp RNA-binding attenuation protein AT = anti-TRAP

Figure 17-14 (a) Model of the trp RNA-binding attenuation protein (TRAP). The symmetrical molecule consists of 11 subunits, each capable of binding to one molecule of tryptophan that is embedded within the subunit. (b) The interaction of a tryptophan-bound TRAP molecule with the leader sequence of the trp operon of B. subtilis. This interaction induces the formation of the terminator hairpin, attenuating expression of the trp operon. Figure 17.14

Figure 17-14a (a) Model of the trp RNA-binding attenuation protein (TRAP). The symmetrical molecule consists of 11 subunits, each capable of binding to one molecule of tryptophan that is embedded within the subunit. (b) The interaction of a tryptophan-bound TRAP molecule with the leader sequence of the trp operon of B. subtilis. This interaction induces the formation of the terminator hairpin, attenuating expression of the trp operon. Figure 17.14a

Figure 17-14b (a) Model of the trp RNA-binding attenuation protein (TRAP). The symmetrical molecule consists of 11 subunits, each capable of binding to one molecule of tryptophan that is embedded within the subunit. (b) The interaction of a tryptophan-bound TRAP molecule with the leader sequence of the trp operon of B. subtilis. This interaction induces the formation of the terminator hairpin, attenuating expression of the trp operon. Figure 17.14b

17.8 The ara Operon Is Controlled by a Regulator Protein That Exerts Both Positive and Negative Control

Figure 17-15 Genetic regulation of the ara operon Figure 17-15 Genetic regulation of the ara operon. The regulatory protein of the araC gene acts as either an inducer (in the presence of arabinose) or a repressor (in the absence of arabinose). Figure 17.15

Figure 17-15a Genetic regulation of the ara operon Figure 17-15a Genetic regulation of the ara operon. The regulatory protein of the araC gene acts as either an inducer (in the presence of arabinose) or a repressor (in the absence of arabinose). Figure 17.15a

Figure 17-15b Genetic regulation of the ara operon Figure 17-15b Genetic regulation of the ara operon. The regulatory protein of the araC gene acts as either an inducer (in the presence of arabinose) or a repressor (in the absence of arabinose). Figure 17.15b

Figure 17-15c Genetic regulation of the ara operon Figure 17-15c Genetic regulation of the ara operon. The regulatory protein of the araC gene acts as either an inducer (in the presence of arabinose) or a repressor (in the absence of arabinose). Figure 17.15c