Bacterial Physiology (Micr430)
Lecture 12
Bacterial Genetics
(Based on Snyder and Champness Book)
Definitions you should know
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Conjugation
Transformation
Transduction
Merodiploid
Plasmid
cis-acting vs trans-acting
PLASMIDS
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A plasmid that can mediate its own
transfer to a new strain - conjugative
plasmid; otherwise it’s nonconjugative.
Plasmid partitioning - a process that
ensures proper distribution of replicated
plasmids into daughter cells.
Two plasmids belonging to the same
incompatibility group cannot be
maintained stably.
PLASMIDS
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Many plasmids harbor resistance (R)
factors that confer resistance to
antibiotics.
Most plasmids are circular;
Streptomyces have linear plasmids
Plasmid replication:
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Theta type
rolling circle
CONJUGATION
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F factor is the prototype conjugative
plasmid.
F factor has genes coding for sex pili
formation;
Presence of sex pili is correlated with
the ability of the cell to serve as a
donor of genetic material
oriT is origin of transfer
Hfr formation
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F factor can exist autonomously in a cell as a
plasmid or can integrate into the bacterial
chromosome.
A cell that contains an integrated F factor is
called Hfr cell (high frequency of
recombination)
An integrated F factor still has transfer
functions; a conjugation with an F- strain will
transfer host DNA
Host genetic map can be obtained
TRANSFORMATION
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Natural Transformation occurs in many
bacterial genera including Streptococcus,
Haemophilus, Neisseria, Xanthomonas,
Rhizobium, Bacillus and Staphylococcus.
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Competence is defined as a physiological
state that permits a cell to take up
transforming DNA and be genetically changed
by it.
Natural transformation
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Natural competence discovery - the Griffith experiment
Gram positive transformation
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In Streptococcus pneumoniae, the
competence state is transient and persists for
only a short period of time during late
exponential phase.
It is induced by competence-stimulating
peptide (CSP); binding of this activator
protein to receptors on the plasma membrane
triggers synthesis of many new proteins
Cells develop capacity to bind DNA molecules
Gram negative transformation
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Neisseria gonorrheae and Haemophilus
influenzae are naturally competent, but only
during stationary phase.
In Neisseria gonorrheae, DNA first binds
nonspecifically to cell surface. Then an
unknown protein or protein complex
recognizes a specific sequence
(GCCGTCTGAA) within the bound DNA
Transport of the tagged DNA across the outer
membrane involves a type IV pilus structure
made up by PilE protein.
Gram negative transformation
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Once in the periplasm, donor DNA needs help
from ComL and Tpc to get through murein
layer.
An inner membrane protein ComA helps to
transport donor DNA across the inner
membrane
Forced Competence
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Organisms not considered naturally
transformable (E. coli and S. typhimurium)
can be transformed under special laboratory
conditions
These include CaCl2 treatment of cells or
electrical shock (electroporation) to transfer
plasmid DNA into the cells
TRANSDUCTION
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There are two types of transduction:
Generalized: phage-mediated transfer of any
portion of a donor cell’s genome into a
second host.
Specialized transduction is mediated by
bacteriophage that integrate into a specific
site on the bacterial chromosome; this
specificity limits transfer of genetic material to
host markers that lie within immediate vicinity
of this site.
RECOMBINATION
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There are two types of recombinations:
General or RecA-dependent recombination
requires a large region of homology between
donor and recipient DNAs.
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Occurs more frequent
RecA-independent or nonhomologous
recombination requires very little sequence
homology.
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Occurs rarely
Requires special proteins
General Recombination:
requirements
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Identical or very similar sequences in the
cross-over region
Complementary base pairing between
double-stranded DNA molecules
Recombination enzymes
Heteroduplex formation
Models of recombination
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Holliday double-strand invasion model
Migration of Holliday junctions
Single-strand
Invasion
Model
Transposable elements
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Transposable elements are discrete
sequences of DNA that encode functions to
catalyze the movement of the transposable
element from one DNA site to a second.
Two types of transposition:
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Replicative: involving both replication and
recombination with a copy of the element
remaining at the original site
Conservative: no replication involved; the element
is simply moved to a new location
Transposable elements classes
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Insertion sequences (IS), encoding no
function other than transposition, with
inverted repeats at either of its ends
Composite transposons – formed by
two IS elements of the same type,
bracketing other genes such as drug
resistance
Composite transposons