One of the main lessons of this chapter is that severalbacterial genes are often transcribed from a singlepromoter into a large multigene (polycistronic) transcript. The region of DNA containing the set of genesthat are cotranscribed, along with all of the regulatoryelements that control the expression of these genes, iscalled an operon.a. Which of the mechanisms in the following listcould explain differences in the levels of themRNAs for different operons?b. Which of the mechanisms in the following listcould explain differences in the levels of theprotein products of different genes in the sameoperon?i. Different promoters might have different DNAsequences.ii. Different promoters might be recognized bydifferent types of RNA polymerase.iii. The secondary structures of mRNAs might differ so as to influence the rate at which they aredegraded by ribonucleases.PROBLEMSProblems 575a. Why did the mating with a nonlysogenic recipientresult in phage growth and release, but the infectionof a lysogenic recipient did not?b. Explain how this phenomenon relates to thePaJaMo experiment in Fig. 16.6.c. Explain how this phenomenon relates to hybriddysgenesis, described in Problem 29 of Chapter 13.11. Mutants were isolated in which the constitutive phenotype of a missense lacI mutation was suppressed.That is, the operon was now inducible. These suppressor mutations mapped to the operon, not to the lacIgene. What could these mutations be?12. Suppose you have six strains of E. coli. One is wildtype, and each of the other five has a single one of thefollowing mutations: lacZ−, lacY−, lacI−, oc, andlacIS. For each of these six strains, describe thephenotype you would observe using the following assays. [Notes: (1) IPTG is a colorless synthetic molecule that acts as an inducer of lac operon expressionbut cannot serve as a carbon source for bacterialgrowth because it cannot be cleaved byβ-galactosidase; (2) X-gal cannot serve as a carbonsource for growth; (3) E. coli requires active lactosepermease (the product of lacY) to allow lactose,X-gal, or IPTG into the cells.]a. Growth on medium in which the only carbonsource was lactose.b. Colony color in medium containing glycerol as theonly carbon source, X-gal, and IPTG.c. Colony color in medium containing glycerol as theonly carbon source and X-gal, but no IPTG.d. Colony color in medium containing high levels ofglucose as the only carbon source, X-gal, andIPTG.e. Colony color in medium containing high levels ofglucose as the only carbon source and X-gal, butno IPTG.13. The previous problem raises some interesting issues:a. In most experiments using the lac operon, researchers use the synthetic inducer IPTG to turn onoperon expression, instead of lactose or allolactose.What do you think is the advantage of using IPTG?b. Scientists were originally puzzled by what theytermed the lactose paradox. To turn on expressionof the lac operon, an inducer (whether IPTG orlactose/allolactose) needs to be able to get into thecell. Import of this inducer requires the presence ofthe Lac permease enzyme in the cell membrane(Fig. 16.2). But if the lac operon is being repressedprior to addition of the inducer, no Lac permeaseshould be present, so no inducer could be imported,and induction could never occur. Yet inductionobviously does occur; how might this be possible?7. You are studying an operon containi iv. In an operon, some genes are farther away fromthe promoter than other genes.v. The translational initiation sequences at thebeginning of different open reading frames inan operon might result in different efficienciesof translation.vi. Proteins encoded by different genes in anoperon might have different stabilities
Gene Interactions
When the expression of a single trait is influenced by two or more different non-allelic genes, it is termed as genetic interaction. According to Mendel's law of inheritance, each gene functions in its own way and does not depend on the function of another gene, i.e., a single gene controls each of seven characteristics considered, but the complex contribution of many different genes determine many traits of an organism.
Gene Expression
Gene expression is a process by which the instructions present in deoxyribonucleic acid (DNA) are converted into useful molecules such as proteins, and functional messenger ribonucleic (mRNA) molecules in the case of non-protein-coding genes.
One of the main lessons of this chapter is that several
bacterial genes are often transcribed from a single
promoter into a large multigene (polycistronic) transcript. The region of DNA containing the set of genes
that are cotranscribed, along with all of the regulatory
elements that control the expression of these genes, is
called an operon.
a. Which of the mechanisms in the following list
could explain differences in the levels of the
mRNAs for different operons?
b. Which of the mechanisms in the following list
could explain differences in the levels of the
protein products of different genes in the same
operon?
i. Different promoters might have different DNA
sequences.
ii. Different promoters might be recognized by
different types of RNA polymerase.
iii. The secondary structures of mRNAs might differ so as to influence the rate at which they are
degraded by ribonucleases.
PROBLEMS
Problems 575
a. Why did the mating with a nonlysogenic recipient
result in phage growth and release, but the infection
of a lysogenic recipient did not?
b. Explain how this phenomenon relates to the
PaJaMo experiment in Fig. 16.6.
c. Explain how this phenomenon relates to hybrid
dysgenesis, described in Problem 29 of Chapter 13.
11. Mutants were isolated in which the constitutive
That is, the operon was now inducible. These suppressor mutations mapped to the operon, not to the lacI
gene. What could these mutations be?
12. Suppose you have six strains of E. coli. One is wild
type, and each of the other five has a single one of the
following mutations: lacZ−, lacY−, lacI−, oc
, and
lacIS
. For each of these six strains, describe the
phenotype you would observe using the following assays. [Notes: (1) IPTG is a colorless synthetic molecule that acts as an inducer of lac operon expression
but cannot serve as a carbon source for bacterial
growth because it cannot be cleaved by
β-galactosidase; (2) X-gal cannot serve as a carbon
source for growth; (3) E. coli requires active lactose
permease (the product of lacY) to allow lactose,
X-gal, or IPTG into the cells.]
a. Growth on medium in which the only carbon
source was lactose.
b. Colony color in medium containing glycerol as the
only carbon source, X-gal, and IPTG.
c. Colony color in medium containing glycerol as the
only carbon source and X-gal, but no IPTG.
d. Colony color in medium containing high levels of
glucose as the only carbon source, X-gal, and
IPTG.
e. Colony color in medium containing high levels of
glucose as the only carbon source and X-gal, but
no IPTG.
13. The previous problem raises some interesting issues:
a. In most experiments using the lac operon, researchers use the synthetic inducer IPTG to turn on
operon expression, instead of lactose or allolactose.
What do you think is the advantage of using IPTG?
b. Scientists were originally puzzled by what they
termed the lactose paradox. To turn on expression
of the lac operon, an inducer (whether IPTG or
lactose/allolactose) needs to be able to get into the
cell. Import of this inducer requires the presence of
the Lac permease enzyme in the cell membrane
(Fig. 16.2). But if the lac operon is being repressed
prior to addition of the inducer, no Lac permease
should be present, so no inducer could be imported,
and induction could never occur. Yet induction
obviously does occur; how might this be possible?
7. You are studying an operon containi iv. In an operon, some genes are farther away from
the promoter than other genes.
v. The translational initiation sequences at the
beginning of different open reading frames in
an operon might result in different efficiencies
of translation.
vi. Proteins encoded by different genes in an
operon might have different stabilities
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