Regulation of Gene Expression in Prokaryotes Chapter 17 Regulation of Gene Expression in Prokaryotes Copyright © 2009 Pearson Education, Inc.

Prokaryotes Respond to Environmental Conditions Constitutive enzymes Adaptive enzymes Inducible Repressible Regulation, whether inducible or repressible, can be: Negative Positive

Operon Genes that code for related enzymes tend to be clustered together Under control of a single regulatory region Regulatory region is cis and located upstream The genes and the regulatory region are called an operon

Common Operons in Prokaryotes The Lac Operon - Inducible The Trp Operon - Repressible The Ara Operon – positive and negative control

The Lac Operon 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

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

How was the operon discovered? Using a gratuitous inducer IPTG – Isopropylthiogalactoside Constitutive mutants

IPTG Figure 17-4 The gratuitous inducer isopropylthiogalactoside (IPTG). Figure 17.4

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

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

CAP – Catabolite Activating Protein 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

cAMP Figure 17-9 The formation of cAMP from ATP, catalyzed by adenyl cyclase. Figure 17.9

The Trp Operon 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

Attenuation A type of repression where transcription is greatly reduced, but not necessarily entirely In the Trp Operon, a leader sequence is transcribed The mRNA can form one of two hairpins Terminator hairpin – when tryptophan is present Antiterminator hairpin – when tryptophan is absent

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

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