In negative control the binding of regulatory protein to DNA causes

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Part Four: Gene Regulation

Section I =  Chapter 15

POSITIVE AND NEGATIVE CONTROL SHOWN BY THE lac OPERON OF E. COLI

      A.    Definitions and general comments

1.    Operons

An operon is a cluster of coordinately regulated genes.  It includes structural genes [generally encoding enzymes], regulatory genes [encoding, e.g. activators or repressors] and regulatory sites [such as promoters and operators].

2.    Negative versus positive control

a.   The type of control is defined by the response of the operon when no regulatory protein is present.

b.   In the case of negative control, the genes in the operon are expressed unless they are switched off by a repressor protein.  Thus the operon will be turned on constitutively [the genes will be expressed] when the repressor in inactivated.

c.   In the case of positive control, the genes are expressed only when an active regulator protein, e.g. an activator, is present.  Thus the operon will be turned off when the positive regulatory protein is absent or inactivated.

Table 4.1.1.  Positive vs. negative control

3.   Catabolic versus biosynthetic operons

a.   Catabolic pathways catalyze the breakdown of nutrients [the substrate for the pathway] to generate energy, or more precisely ATP, the energy currency of the cell.  In the absence of the substrate, there is no reason for the catabolic enzymes to be present, and the operon encoding them is repressed.  In the presence of the substrate, when the enzymes are needed, the operon is induced or de-repressed.

Table 4.1.2. Comparison of catabolic and biosynthetic operons

      For example, the lac operon encodes the enzymes needed for the uptake [lactose permease] and initial breakdown of lactose [the disaccharide b-D-galactosyl-1->4-D-glucose] into galactose and glucose [catalyzed by b-galactosidase].  These monosaccharides are broken down to lactate [principally via glycolysis, producing ATP], and from lactate to CO2 [via the citric acid cycle], producing NADH, which feeds into the electron-transport chain to produce more ATP [oxidative phosphorylation].  This can provide the energy for the bacterial cell to live.  However, the initial enzymes [lactose permease and b-galactosidase] are only needed, and only expressed, in the presence of lactose and in the absence of glucose.  In the presence of the substrate lactose, the operon in turned on, and in its absence, the operon is turned off.

b.   Anabolic, or biosynthetic, pathways use energy in the form of ATP and reducing equivalents in the form of NAD[P]H to catalyze the synthesis of cellular components [the product] from simpler materials, e.g. synthesis of amino acids from small dicarboxylic acids [components of the the citric acid cycle].  If the cell has plenty of the product already [in the presence of the product], the the enzymes catalyzing its synthesis are not needed, and the operon encoding them is repressed.  In the absence of the product, when the cell needs to make more, the biosynthetic operon is induced.

      E.g., the trp operon encodes the enzymes that catalyze the conversion of chorismic acid to tryptophan.  When the cellular concentration of Trp [or Trp-tRNAtrp] is high, the operon is not expressed, but when the levels are low, the operon is expressed.

4.   Inducible versus repressible operons

a.   Inducible operons are turned on in reponse to a metabolite [a small molecule undergoing metabolism] that regulates the operon.  E.g. the lac operon is induced in the presence of lactose [through the action of a metabolic by-product allolactose].

b.   Repressible operons are switched off in reponse to a small regulatory molecule.  E.g., the trp operon is repressed in the presence of tryptophan.

      Note that in this usage, the terms are defined by the reponse to a small molecule.  Although lac is an inducible operon, we will see conditions under which it is repressed or induced [via derepression].

      Table 4.1.3.

      B.     Map of the E. colilac operon

      Figure 4.1.1.

1.   Promoters = p = binding sites for RNA polymerase from which it initiates transcription.

There are separate promoters for the lacI gene and the lacZYA genes.

2.   Operator = o = binding site for repressor; overlaps with the promoter for lacZYA.

3.   Repressor encoded by lacI gene

4.   Structural genes: lacZYA

lacZ encodes b-galactosidase, which cleaves the disccharide lactose into galactose and glucose.

lacY encodes the lactose permease, a membrane protein that faciltitates uptake of lactose.

lacA encodes b-galactoside transacetylase;  the function of this enzymes in catabolism of lactose is not understood [at least by me].

C.  Negative control

The lac operon is under both negative and positive control.  The mechanisms for these will be considered separately.

1.   In negative control, the lacZYA genes are switched off by repressor when the inducer is absent [signalling an absence of lactose].  When the repressor tetramer is bound to o, lacZYA is not transcribed and hence not expressed.

      Figure 4.1.2.  Repressed lac operon

2.   When inducer is present [signalling the presence of lactose], it binds the repressor protein, thereby altering its conformation, decreasing its affinity for o, the operator. The dissociation of the repressor-inducer complex allows lacZYA to be transcribed and therefore expressed.

      Figure 4.1.3.  Induction of the lac operon by derepression.

D.  Inducers

1.   The natural inducer [or antirepressor], is allolactose, an analog of lactose.  It is made as a metabolic by-product of the reaction catalyzed by b-galactosidase.  Usually this enzyme catalyzes the cleavage of lactose to galactose + glucose, but occasionally it will catalyze an isomerization to form allolactose, in which the galacose is linked to C6 of glucose instead of C4.

2.   A gratuitous inducer will induce the operon but not be metabolized by the encoded enzymes; hence the induction is maintained for a longer time.  One of the most common ones used in the laboratory is a synthetic analog of lactose called isopropylthiogalactoside [IPTG].  In this compound the b-galactosidic linkage is to a thiol, which is not an efficient substrate for b-galactosidase.

E.  Regulatory mutants

Regulatory mutations affect the amount of all the enzymes encoded by an operon, whereas mutations in a structural gene affects only the activity of the encoded [single] polypeptide.

1.   Repressor mutants

a.   Wild-type strains [lacI+] are inducible.

b.   Most strains with a defective repressor [lacI-] are constitutive, i.e. they make the enzymes encoded by the lac operon even in the absence of the inducer.

c.   Strains with repressor that is not able to interact with the inducer [lacIS] are noninducible.  Since the inducer cannot bind, the repressor stays on the operator and prevents expression of the operon even in the presence of inducer.

d.   Deductions based on phenotypes of mutants

Table 4.1.4.  Phenotypes of repressor mutants