Chapter 8:Bacterial Genetics

Important Point:

Bac

teria

l Gen

etic

s “Acquiring genes through gene transfer provides new genetic information to microorganisms, which may allow them to survive changing environments.”

“The major source of variation within a bacterial species is mutation.”

“In mutations, usually only a single gene changes at any one time.”

“In contrast, gene transfer results in many genes being transferred simultaneously, giving the recipient cell much more additional genetic information.”

Bac

teria

l Gen

etic

s O

verv

iew Most bacteria are haploid which means that there is

no such thing as dominance-recessive relationships among bacterial alleles.

Bacteria don’t have sex in the animal/plant sense of sex (i.e., mating followed by recombination of whole genomes).

Instead, bacteria acquire DNA from other bacteria through three distinct mechanisms: Transformation Transduction Conjugation

This DNA may or may not then recombine into the recipient’s genome.

We use phrases like “Lateral” or “Horizontal” Gene Transfer to describe these sexual interactions.

Bacterial DNA is also subject to mutation, damage (not the same thing as mutation), and natural selection.

Mut

atio

n: T

erm

s &

Con

cept

s Wild Type refers to the microorganism as isolated from the wild.

A mutated microorganism that has lost a metabolic function, particularly an ability to synthesize a specific growth factor, is called an Auxotroph.

The wild-type parent to an auxotroph is called a Prototroph.

A Mutation is found in a gene; a mutant is an organism harboring a Mutation.

We designate mutant phenotypes using three-letter abbreviations; the phenotype of a tryptophan-requiring auxotroph would be described as Trp-.

A bacterium that has mutated to resistance to an antibiotic (or other substance) is given the superscript “R”; thus, the phenotype ampicillin resistance is indicated as AmpR.

Mutants can be spontaneous or induced by a Mutagen; an agent that causes DNA to mutate.

Typ

es o

f Mut

atio

ns Base Substitution Point mutation = single base is substituted. Missense mutation = base change changes single

amino acid to different amino acid. Nonsense mutation = base change changes single

amino acid to stop codon. Null or Knockout mutation = mutation that totally

inactivates a gene. Deletion or insertion mutation = change in number of

bases making up a gene. Frameshift mutation = insertion or deletion of

something other than multiples of three bases. Frameshifts typically radically change downstream

codons, generating stop codons, and typically knocking out gene function.

Reversion mutation = mutated change back to that of wild type.

Rat

es o

f Mut

atio

n The mutation rate of different genes usually varies

between 10-4 and 10-12 mutations per cell division (essentially equivalent to per cell).

10-4 = one in 10,000; 10-12 = one in one trillion. To calculate the probability of two independent

mutations we multiple the two mutation rates. Thus, if streptomycin resistance occurs at a rate of 10-6

mutations per cell division and the rate of mutation to resistance to penicillin is 10-8 then the rate of mutation to both antibiotics is 10-6 * 10-8 = 10-14 (note that the exponents are added).

That is, we would have to have a population of one-hundred trillion cells to have one double mutant, which even for bacteria is a lot of cells.

This is the basis for Combination Therapy, e.g., the use of more than one chemotherapeutic against tuberculosis, HIV, cancer, etc.

The odds of sufficiently multiply resistant mutants drops with each new chemotherapeutic added to the mix.

Dire

ct S

elec

tion

for

Mut

ants

Indi

rect

Sel

ectio

n: R

eplic

a P

latin

g

Indi

rect

Sel

ectio

n:P

enic

illin

Enr

ichm

ent

Indi

rect

Sel

ectio

n:Is

olat

ion

of ts

Mut

ants

This is one example of isolation of mutants carrying conditionally lethal mutations found in essential genes.

Ames Salmonella Test

DN

A-M

edia

ted

Tra

nsfo

rmat

ion

Note that DNA is taken up naked from

the environment.

DN

A-M

edia

ted

Tra

nsfo

rmat

ion

Orig

inal

Tra

nsfo

rmat

ion

Exp

.F

. Grif

fith

(192

8) u

sing

pne

umoc

occi

Artificial Competenceby Electroporation

Competence denotes the

ability to take up DNA naked

from the environment.

Most bacteria are not naturally competent but many can be

made artificially so.

Artificially induced

competence is very important

to gene cloning.

Gen

eral

ized

Tra

nsdu

ctio

n

Bacteriophages are viruses that only infect (and

can kill) bacteria.

Gen

eral

ized

Tra

nsdu

ctio

n

Con

juga

tion:

Sex

or

F P

ilus

Con

juga

tion:

F P

lasm

id T

rans

fer

F a

nd O

ther

Pla

smid

s F plasmids encode genes that allow both their

replication and transfer. They are thus known as Self-Transmissible Plasmids. There are other plasmids that can take advantage of

conjugation but don’t encode the the necessary genes. These are non-self transmissible plasmids.

Transduction is also capable of transferring smaller plasmids.

R plasmids are named not for their mode of transmission but instead for the resistance genes that they encode such as to antibiotics.

Some plasmids are present in bacteria in low copy numbers (1 or 2/bacterium) whereas other plasmids are present in high copy numbers (such 100s/bact.).

Plasmids, R and otherwise, can have very wide host ranges allowing easy transfer of already evolved genes between bacterial species.

Sel

f-T

rans

mis

sibl

e R

Pla

smid

Resistance Transfer Factor (conjugation genes)

Note multiple

resistance genes.

Tra

nsfe

r of

non

-R V

irule

nce

Fac

tors

Genes that can make bacteria more virulent (able to cause disease) are called Virulence Factor genes.

Virulence factors include fimbriae that allow attachment to host tissues, exotoxins, etc.

Virulence factor genes may be transferred by transformation, transduction, or conjugation.

Virulence factor genes tend to congregate on bacterial chromosomes in regions known as Pathogenicity Islands.

New bacterial pathogens can emerge via the uptake of entire pathogenicity islands transferred intact from unrelated bacteria.

Tra

nsfe

r P

rote

ctio

n: R

-M S

yste

ms Not all incoming DNA is necessarily good for the

receiving bacterium (i.e., DNA can be parasitic). Bacteria employ Restriction Enzymes to protect

themselves from the foreign DNA. Restriction enzymes recognize specific,

palindromic (same spelling backward and forward) DNA sequences of 4 to 8 base pairs in length that are known as Recognition Sequences.

Bacteria also employ Modification Enzymes that modify DNA to protect it from Restriction Enzymes.

Together these are called Restriction-Modification Systems.

Restriction enzymes are crucial components of genetic engineering.

Res

tric

tion

End

onuc

leas

e A

ctio

n

Res

tric

tion

End

onuc

leas

e A

ctio

n

Note in particular that DNA is cut at palindromic

regions.

DN

A M

odifi

catio

n: R

E P

rote

ctio

n

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