Download Viruses as Pathogens in Bacterial Gene Regulation

Viruses as Pathogens in Bacterial Gene
Regulation
• Bacterial DNA
– single double-stranded circular DNA
• densely supercoiled in a region called the nucleoid region
• replication occurs in both directions from the replication fork
(no Okazaki)
– replication via binary fission (splitting)
– asexual process
» offspring are clones
• very quick replication time ~20min
– 1 E. coli can create a colony of 1 x108 in 12 hours
Sources of Genetic Variation
• mutations are the main
source of variation (~9
million/day)
• transformation
– process by which bacteria
pick up DNA from similar
species and incorporate
into their genome
– in biotechnology E. coli can
be induced into
transduction with the
addition of Ca+ to the
growth medium
• transduction
– bacteriophage introduces
new DNA into the bacteria
Sources of Genetic Variation
• conjugation
– bacteria sex!
– bacteria containing the F-factor (fertility) forms a pili
» long extension that pulls another bacteria close
» a cytoplasmic bridge is established and DNA is passed from the
bacterium with the F factor (F+) to the recipient (F-)
– F-factor may be located in a separate circular region called a plasmid
» if the plasmid reinserts itself into the DNA is it called an episome
» an episome is an example of a transposable element - a gene that
can be relocated• called transposons if they are larger segments
• insertion segments are inverted repeats that act as boundaries
that separate transposed genes
» If the conjugation infers resistance to antibiotics it is renamed to Rfactor (resistance)
Lac Operon Concept
• works by the process of feedback inhibition
– product of the reaction blocks a step in the
metabolic pathway leading to the product
• shuts off the production at the DNA level - inhibits the
production of mRNA
• saves resources of the organism
• groups of enzymes that are
grouped into the same mRNA
transcript (ex: trp - gene for
making tryptophan)
– each protein has its own start
and stop sequence
– benefit: single on & off switch
for entire process
• switch is called the operator
– located within the promoter
region
– can be switched off by the
presence of a repressor bound
to the operator
» blocks attachment of the
RNA Pol
» product of a repressor
gene that is some
distance from the operon
(trpR)
» Repressors are reversible
- only active if the
corepressor is present to
its ligand trp (allosteric
inhibition)
Operons
Inducible Repressor
• the presence of an
inducer inactivates
repressor
– ex: lactose
• allolactose binds to the
repressor changing its
conformation
• the repressor is
released
• suppression of enzyme
for the uptake saves
resources
– negative control
mechanism
Positive Gene regulation
• CAP promoter binds with cAMP stimulates the transcription in the
absence of glucose (its preferred food)
– called an activator