Chapter 16.2, Problem 19AQP

Summary Introduction

To determine:

The lac genotype by using plus sign (+) to indicate the synthesis of $\text{β-}\text{galactosidase}$ and permease and a minus sign (-) to indicate no synthesis of proteins.

Introduction:

An operon is defined as a group of genes that have a common promoter and regulator and also transcribed as a single unit. This transcription results in the production of single $\text{mRNA}$ which encodes several types of proteins. The gene of lac operon gets activated in the presence of lactose sugar.

Explanation of Solution

Tabular representation: Table 1: Transcription of a gene in the present or absent of lactose.

Genotype of strain	Lactose absent		Lactose present
	$\text{β-Galactosidase}$	Permease	$\text{β-Galactosidase}$	Permease
$lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}$	-	-	+	+
$lac{I}^{-}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}$	+	+	+	+
$lac{I}^{+}lac{P}^{+}lac{O}^{c}lac{Z}^{+}lac{Y}^{+}$	+	+	+	+
$lac{I}^{-}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{-}$	+	-	+	-
$lac{I}^{-}lac{P}^{-}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}$	-	-	-	-
$\frac{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{-}lac{Y}^{+}}{lac{I}^{-}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{-}}$	-	-	+	+
$\frac{lac{I}^{-}lac{P}^{+}lac{O}^{c}lac{Z}^{+}lac{Y}^{+}}{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{-}lac{Y}^{-}}$	+	+	+	+
$\frac{lac{I}^{-}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{-}}{lac{I}^{+}lac{P}^{-}lac{O}^{+}lac{Z}^{-}lac{Y}^{+}}$	-	-	+	-
$\frac{lac{I}^{+}lac{P}^{-}lac{O}^{c}lac{Z}^{-}lac{Y}^{+}}{lac{I}^{-}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{-}}$	-	-	+	-
$\frac{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}}{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}}$	-	-	+	+
$\frac{lac{I}^{s}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{-}}{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{-}lac{Y}^{+}}$	-	-	-	-
$\frac{lac{I}^{s}lac{P}^{-}lac{O}^{+}lac{Z}^{-}lac{Y}^{+}}{lac{I}^{+}lac{P}^{+}lac{O}^{+}lac{Z}^{+}lac{Y}^{+}}$	-	-	-	-

Table 1: Transcription of a gene in the present or absent of lactose.

Lac operon is defined as an operon which is required for lactose metabolism and its transport. Lac operon is an example of the negative inducible operon. This operon contains three types of a structural gene which are designated as lac Z, lac Y, and lac A, that contain a common promoter and an operator region. Lac operon also contains regulator gene which is designated as lac I. Lac I gene encodes a repressor protein and inhibit transcription of a gene. Mutation in any genes presents in operon cause the change in gene expression.

The presence and absence of an enzyme in the given strain genotypes are listed below:

(i) In wild-type bacterial strain, the regulation of the lac operon gene is dependent on the presence and absence of lactose (substrate). If lactose is present, then the transcription of the structural gene will turn on and the synthesis of an enzyme $\text{β-}\text{galactosidase}$ and permease would take place. If lactose is absent, then the transcription of a structural gene will not turn on and synthesis of an enzyme $\text{β-}\text{galactosidase}$ and permease does not take place.

(ii) Mutation in any structural gene results in a change of their gene expression. Suppose, a mutation in $lacI$ gene will be represented as $lac{I}^{-}$, then it will repressed the gene expression. In this case $lacI$ gene unable to synthesize the repressor protein, which results in the constitutive transcription of a structural gene. Thus, in second strain genotype gene for synthesis of $\text{β-}\text{galactosidase}$ and permease will continuously turn on.

(iii) If a mutation occurs at the site of an operator, then the complex of repressor and corepressor protein will not bind to the operator and transcription will continuously take place. In this case, repressor protein would not able to turn off the gene expression. Thus, in third strain, the gene for $\text{β-}\text{galactosidase}$ and permease enzyme synthesis will continuously turn on.

(iv) Mutation in regulator gene prevents the synthesis of the repressor protein. Thus, the transcription of the structural gene will continuously turn on. But in the given fourth strain gene for synthesis of permease enzyme is also mutated so, permease enzyme will not synthesize in both conditions (presence of lactose and absence of lactose).

(v) If a mutation occurs at regulator region, then the regulator gene unable to synthesize the repressor protein this result in the constitutive transcription of a structural gene. But in the given fifth strain promoter region is also mutated which prevent the binding of RNA polymerase to operator and enzyme fail to start transcription of a structural gene. Thus, in fifth strain, no enzyme will produce in both conditions (presence of lactose and absence of lactose).

(vi) If a mutation occurs in regulator gene present in one DNA strand and other DNA strand regulator gene is not mutated $\left(\frac{lac{I}^{+}}{lac{I}^{-}}\right)$, then this condition is characterized as partial-diploid genotype. In this case, the repressor protein encoded by the nonmutated regulator gene will not repress the transcription of a structural gene. This results in the synthesis of an enzyme only in presence of substrate.

(vii) The mutation in the operator gene represented by $lac{O}^c$ and that determines the mutation is dominant. Hence, the synthesis of repress protein will not repress the transcription process. Thus, the synthesis of an enzyme $\text{β-}\text{galactosidase}$ and permease will continuously take place.

(viii) As the regulator gene is trans-acting, so if a mutation occurs in one DNA strand of regulator gene will not repress the transcription of other DNA gene. But in this case, permease gene is also mutated, so the synthesis of permease enzyme will not occur either in the presence and absence of lactose. Thus, in this case, only synthesis of $\text{β-}\text{galactosidase}$ enzyme will occur in the presence of substrate.

(ix) In the ninth strain mutation in operator gene is dominant, thus repressor protein will not repress the transcription. But in this case gene for the synthesis of permease enzyme is also mutated. So, enzyme permease will not form in both conditions (presence of lactose and absence of lactose). $\text{β-}\text{galactosidase}$ will be synthesized only in presence of lactose.

(x) The tenth strain is wild type; hence transcription of a structural gene would occur only in presence of substrate. In absence of substrate, no enzyme will be produced.

(xi) The mutation represented by $lac{I}^s$ is super dominant; repressor protein encoded by $lac{I}^s$ will repress the transcription process in both strand of DNA. Thus, no enzyme will be synthesized in both conditions (presence of lactose and absence of lactose).

(xii) The mutation represented by $lac{I}^s$ is super dominant; repressor protein encoded by $lac{I}^s$ will repress the transcription process in both strand of DNA. Thus, no enzyme will be synthesized in both conditions (presence of lactose and absence of lactose).

Conclusion

The enzyme synthesized by the structural gene will depend on the presence or absence of a substrate. If lactose is present, then the gene for lac operon will turn on and if lactose is absent then gene for lac operon will turn off. Mutation in any of the lac operon gene will change the gene expression.