Microbiology, 2021

11 • Summary 459
undamaged DNA strand as a template. Bacteria
and other organisms may also use direct repair,
in which the photolyase enzyme, in the presence
of visible light, breaks apart the pyrimidines.
• Through comparison of growth on the complete
plate and lack of growth on media lacking
specific nutrients, specific loss-of-function
mutants called auxotrophs can be identified.
• The Ames test is an inexpensive method that
uses auxotrophic bacteria to measure
mutagenicity of a chemical compound.
Mutagenicity is an indicator of carcinogenic
potential.
11.6 How Asexual Prokaryotes
Achieve Genetic Diversity
• Horizontal gene transfer is an important way
for asexually reproducing organisms like
prokaryotes to acquire new traits.
• There are three mechanisms of horizontal gene
transfer typically used by bacteria:
transformation, transduction, and
conjugation.
• Transformation allows for competent cells to
take up naked DNA, released from other cells on
their death, into their cytoplasm, where it may
recombine with the host genome.
• In generalized transduction, any piece of
chromosomal DNA may be transferred by
accidental packaging of the degraded host
chromosome into a phage head. In specialized
transduction, only chromosomal DNA adjacent
to the integration site of a lysogenic phage may
be transferred as a result of imprecise excision
of the prophage.
• Conjugation is mediated by the F plasmid,
which encodes a conjugation pilus that brings
an F plasmid-containing F + cell into contact
with an F - cell.
• The rare integration of the F plasmid into the
bacterial chromosome, generating an Hfr cell,
allows for transfer of chromosomal DNA from
the donor to the recipient. Additionally,
imprecise excision of the F plasmid from the
chromosome may generate an F’ plasmid that
may be transferred to a recipient by
conjugation.
• Conjugation transfer of R plasmids is an
important mechanism for the spread of
antibiotic resistance in bacterial communities.
• Transposons are molecules of DNA with
inverted repeats at their ends that also encode
the enzyme transposase, allowing for their
movement from one location in DNA to another.
Although found in both prokaryotes and
eukaryotes, transposons are clinically relevant
in bacterial pathogens for the movement of
virulence factors, including antibiotic resistance
genes.
11.7 Gene Regulation: Operon
Theory
• Gene expression is a tightly regulated process.
• Gene expression in prokaryotes is largely
regulated at the point of transcription. Gene
expression in eukaryotes is additionally
regulated post-transcriptionally.
• Prokaryotic structural genes of related function
are often organized into operons, all controlled
by transcription from a single promoter. The
regulatory region of an operon includes the
promoter itself and the region surrounding the
promoter to which transcription factors can
bind to influence transcription.
• Although some operons are constitutively
expressed, most are subject to regulation
through the use of transcription factors
(repressors and activators). A repressor binds
to an operator, a DNA sequence within the
regulatory region between the RNA polymerase
binding site in the promoter and first structural
gene, thereby physically blocking transcription
of these operons. An activator binds within the
regulatory region of an operon, helping RNA
polymerase bind to the promoter, thereby
enhancing the transcription of this operon. An
inducer influences transcription through
interacting with a repressor or activator.
• The trp operon is a classic example of a
repressible operon. When tryptophan
accumulates, tryptophan binds to a repressor,
which then binds to the operator, preventing
further transcription.
• The lac operon is a classic example an inducible
operon. When lactose is present in the cell, it is
converted to allolactose. Allolactose acts as an
inducer, binding to the repressor and preventing
the repressor from binding to the operator. This
allows transcription of the structural genes.
• The lac operon is also subject to activation.
When glucose levels are depleted, some cellular
ATP is converted into cAMP, which binds to the
catabolite activator protein (CAP). The cAMP-
CAP complex activates transcription of the lac
operon. When glucose levels are high, its
presence prevents transcription of the lac