Download See Fig. 13.1c

Exam 2…
Mean = 72 (after adding 7 points to all tests)
Bacterial Genetics - we will only discuss
mechanisms of genetic exchange (gene
transfer) in bacteria….
Read pages 323-334 and 203-215.
Q. 19 - answers were messed up (real answer is
c) so part of the 7 points was for that question.
Spend a little time going over the test…..
Transduction…..
Transfer of host DNA via bacteriophage
2 types:
Specialized & Generalized Transduction
Generalized results from the lytic cycle of
certain phage (see Fig. 8.16)
Specialized results from lysogeny followed by
the lytic cycle (Fig. 13.11)
Three modes of genetic exchange:
1. Transduction (figs. 8.16 and 13.11)
2. Transformation (persp. 8.1, pg. 205)
3. Conjugation (Figs. 8.18 - 8.22)
In order to understand transduction
we need to know…..Viruses of
Bacteria (bacteriophage or phage)
Always are naked viruses (no membrane)
why?
Huge variety - One species of bacteria can
have many different phage… Table 13.2
Important for:
1. Genetic transfer (transduction)
2. Control of Bacteria in Nature(read Persp. 13.1 pg. 330)
3. Some can cause bacteria to become
pathogens (e.g. diphtheria - see Table 13.3)
Table 13.2. Note the variety of shapes,
nucleic acid content, and “life cycle”…
See Fig. 13.1c
Host ranges of phage:
Phage are usually very specific to the species
they infect - they attach to specific receptors
on the outer layers of the bacterium e.g. some phage of E. coli attach specifically
to the proteins of the flagellum…
see Figures 13.12 and 8.17
Phage for a particular bacterium also have
DNA methylation patterns like their host and
thus avoid having their DNA cleaved by
restriction enzymes when the DNA enters
the cell…. See Figure 13.13
Bacteriophage
attached to pilus of
E. coli
Fig. 13.12
Virus interactions with host cells or
Replication Cycles of phage…..
See Figs. 13.4 - 13.7
Phage tail fibers
entwined around
flagellum
Phage T4 attached to
specific receptors
(cell wall proteins)
of E. coli.
Fig. 13.4
Fig. 13.04
Fig. 13.5. Example of the lytic cycle: T4 of E.
col
Fig. 13.6. Phage Lambda can undergo a lytic
or the lysogenic cycle
Latent state =
Lysogenic state
Fig. 13.7. Insertion of Lambda into a specific
spot the bacterial chromosome….
Summary of lytic and lysogenic cycles
Consequences of the lysogenic cycle:
Cells are immune to further infection by that
phage
Can lead to specialized transduction (later)
Can cause “lysogenic conversion” = viral genes
that change the phenotype of the host cell - e.g.
some phage have genes for toxin production and
can convert a non-pathogenic bacterium to a
pathogenic one….
See Table 13.3 for examples….
Table 13.3. Lysogenic conversion of bacteria
conferring pathogenic properties….
Back to Bacterial Genetics
Transduction…..
Transfer of host DNA via bacteriophage
2 types:
Prophage = the lysogenic phage incorporated
into the bacterial chromosome…..
Specialized & Generalized Transduction
Generalized results from the lytic cycle of
certain phage (see Fig. 8.16)
Specialized results from lysogeny followed by
the lytic cycle (Fig. 13.11)
Fig. 8.16
Bacterial Genetics
Generalized transduction results from the lytic cycle.
Chromosome is digested into small pieces - some of which end up
being packaged in virus particles…
2. Transformation… uptake and
Fig. 13.11. Specialized transduction by a temperate
phage. Results from the lysogenic cycle… Specific
pieces of DNA (near insertion sites) are transferred…
Fig. 13.11b. Specialized transduction by a temperate
phage
Persp. 8.1, pg. 205. Experiment carried out by Griffin,
showing that something was transferred from dead,
pathogenic, S. pneumoniae to live nonpathogenic cells transforming them into pathogens…
It was later (1944) shown that the “transforming
principle” was DNA (thus studies of transformation led to identification
of DNA as the genetic material in cells).
incorporation (into the chromosome) of
“naked” DNA from the environment….
Expression of this new DNA can alter the
phenotype of the organism, e.g. converting a
non-pathogen into a pathogen….
e.g. Streptococcus pneumoniae… Fig. 19.10. Streptococcus
pneumoniae is pathogenic only when it produces a capsule
(which helps it avoid detection by antibodies and phagocytes)
Fig. 8.14. Transformation of a cell from nonresistant (StrS) to resistant to streptomycin (StrR).
Fig. 8.15. There are many artificial ways to make
bacterial cells “competent” for transformation, e.g.
electroporation, and many chemical treatments that
make temporary holes in the cell wall and membrane
Transformation can take place between even
unrelated bacteria (but it is rarer than
between related because most foreign DNA is
degraded before it can be methylated - see
figure 8.26)
There are also genetic platforms (integrons)
within bacterial chromosomes that can
mobilize large pieces of DNA and integrate
large pieces of foreign DNA behind a
promoter so that they are easily
transcribed…
Bacterial Genetics
Conjugation (bacterial sex) Usually occurs
between a + strain (has a conjugative plasmid)
and a - strain (no plasmid)….
Fig. 8.26
Quick review of Plasmids - Extrachromosomal,
circular, double-stranded DNA molecules.
Plasmids divide and copies go to both daughter cells
during asexual reproduction.
Many plasmids can be transmitted between closely related
species but some are not limited to close relatives.
Not usually essential for a given organism, rather they allow the
organism to adapt to specific environmental conditions.
Therefore, plasmids are often unstable in a host bacterium due
to the increased metabolic load.
Also found in Archaea, Fungi and other euks………
Fig. 8.17. Conjugation between F+ and and F- E. coli…
All sorts of traits can be transferred via
conjugative plasmids - the most relevant traits
for us are antibiotic resistance genes…. But
other important genes are also found on
plasmids.. (Table 8.4)
Antibiotic resistance - usually by coding for an
enzyme that renders the antibiotic non-functional.
Beta-lactamase (inactivates Beta-lactams) is one
example. Penicillin is a Beta-lactam.
Special metabolic properties - some plasmids allow
bacteria to take advantage of situations that might be
otherwise harmful. Breakdown of complex organic
molecules is often plasmid encoded
Table 08.04
The real scary thing about plasmids is that
many of them have the above traits AND
are conjugative as well
Conjugation is brought about via information
stored on fertility plasmids (= conjugative
plasmids)… which contain genes for:
1. The F pilus
2. Genes to mobilize the plasmid (Transfer factors)
3. An origin of replication
See figure 8.22
(Later in lecture…..)
Virulence Plasmids - there are a number of ways that a
plasmid can confer virulence in a bacterium.
1) The production of one or more toxins that
can be directed toward the host or towards other
bacteria (bacteriocins).
2) The ability to form a capsule.
The recent anthrax scare is an example:
Virulent B. anthracis have 2 plasmids that encode for toxin
production and capsule formation. An avirulent strain used
for veterinary vaccination lacks the capsule forming
plasmid.
Other virulence factors on plasmids:
The production of siderophores that enable the bacterium to
scavenge iron in the body (e.g. S. aureus).
Adhesins - proteins or glycoproteins that are usually a
component of capsules or fimbriae that allow the bacterium
to adhere to specific cells.
Fig. 8.18.
Conjugation - transfer of the F plasmid. Note that both cells are
F+ after the mating takes place…..
Fig. 8.19. Hfr formation via integration of the F-plasmid into the
host chromosome at specific insertion sequences (ISs)
Fig. 8.20. Conjugation involving an Hfr cell. Basically the
whole chromosome is now a giant F plasmid - but the whole thing is
rarely transferred..
Fig. 8.21. An F’ plasmid results if some chromosomal DNA is
excised when the plasmid pops back out….
Fig. 8.22.
The regions
of an R
Plasmid.
RTF =
resistance
transfer
factors
R= resistance
genes
Plasmids and a type of conjugation are also
involved in the transfer of genetic information from
bacteria to some Eucaryotes.
Remember Agrobacterium tumefaciens
(a Gm-,
alphaproteobacterium related to Rhizobium Fig.
11.2)
has a Ti plasmid (tumor inducing plasmid) that
codes for:
1. A pilus
2. Transfer factors
3. Transferred DNA (T-DNA) that codes for
plant hormone production and the
production of opines
4. Metabolic genes that allows A. tumefaciens
to eat opines
Persp. 8.2, pg. 211
Movement of DNA within cells
e.g.
transposons
Table 8.3
Transposition involves small segments of DNA
(transposons, "jumping genes") that can move
around chromosomes or plasmids.
e.g. F-plasmids have insertion sequences that
allow the plasmid to integrate into the
chromosome => Hfr cell.
Insertion sequences are very simple and
typically contain only the information needed for
insertion (see figure 8.24). Also used in experiments to disrupt
specific genes.
Transposons (Composite Transposon in Figure 8.24) are more
complex and may contain a number of genes that
confer e.g. antibiotic resistance and/or toxin
production.
Fig. 8.23. Movement of a transposon through a bacterial
community.
Fig. 8.24 a. Insertion sequence, b. Composite transposon
containing an antibiotic resistance gene….
Fig. 08.24