Antibiotics + DNA Flashcards
What are antibiotics?
Major targets?
Disruptors of bacterial structures and functions.
Bacteriostatic = Inhibits growth and reproduction, but doesn’t directly kill.
Bacteriocidal - directly kill bacteria.
Cell-wall synthesis
DNA gyrases
Protein synthesis - 30s or 50s inhibition, tRNA inhibition,
DNA/RNA polymerases.
What are major targets for antibiotics?
Examples:
Cell-wall synthesis:
Penicillins, Beta-lactams, Carbapenems.
DNA gyrase:
Quinolones/Fluoroquinolones.
Protein synthesis - 30s inhibition:
Tetracyclin, streptomycin.
or 50s inhibition:
Erythromycin, Chloramphenicol.
tRNA inhibition:
Puromycin.
DNA/RNA polymerases:
Rifampines
Actinomycin = RNA elongation.
How can you inhibit nucleic acid synthesis?
Quinolones/Fluoroquinolones:
Inhibit DNA polymerisation by antagonising Gyr A subunit of DNA gyrase.
DNA gyrase is a Type II topoisomerase - Heterotetramer.
GyrA subunits involved in cutting and rejoining DNA.
DNA gyrase vital to relax positive supercoils that accumulate ahead of the replication fork in DNA replication.
= Vital for DNA replication.
Facilitates chromosomal condensation by forming negative supercoils.
= Without DNA gyrase, unable to effectively manage DNA topology and will lead to DNA breaks and failure to replicate DNA,
Eukaryotes have other Type II topoisomerases, not DNA gyrase.
How does transcription occur in bacteria/archaea?
Within cytoplasm, since no nucleus.
Promotor region binds RNA polymerase.
Start codon to Stop codon - neither translated.
There are no introns in bacterial genes, only a few in archaeal genes.
Most genes are structured as operons.
-Permitting concerted production of n proteins with related functions.
E.G lactose operon.
Genes are read in the 3’ to 5’ direction by RNA polymerase, because this ensures the mRNA can be read in 5’ to 3’ for translation.
RNA polymerase adds only to 3’ OH group of RNA strand.
How does prokaryotic translation occur?
In what direction are DNA and RNA read?
There is cytoplasmic coupling:
Transcription via numerous RNA polymerases simultaenously transcribing a single gene, with each mRNA translated by polyribosomes.
Genes are read in the 3’ to 5’ direction by RNA polymerase, because this ensures the mRNA can be read in 5’ to 3’ for translation.
RNA polymerase adds only to 3’ OH group of RNA strand.
mRNA read in 5’ to 3’ direction.
What is the structure of RNA polymerase in bacteria?
There is the core Enzyme, made of 5 subunits.
Beta subunit with catalytic site.
The core enzyme can function on its own, but requires a sigma factor to transcribe at the right site.
Sigma factor required for promotor region binding.
= When sigma factor bound, it becomes a HOLOENZYME.
7 sigma factors, correspond to 7 promoters, so there are 7 different holoenzymes.
(In E.Coli)
What is the structure and arrangement of bacterial promotor regions?
Bacterial promoter regions are rich in A and T nucleotide bases.
The promoter sequence is recognised by the sigma factor of RNA polymerase Holoenzyme.
When transcription initiated, sigma factor is released.
no. of sigma factors differes by species, but 7 in E.Coli.
= Allow transcription of gene sets, for heat shock response.
How do RNA polymases differ between bacteria, archaea and eukaryotes?
Bacteria have only 1 Enzyme.
Archaea only have 1 enzyme.
Eukaryotes have 3 different enzymes in total.
There are 5 subunits with the sigma factor added in bacteria.
Archaea can have between 10-15 subunits.
Eukaryotes have 10-16 subunits.
RNA polymerase similarity between eukaryotes and archaea suggest closer evolutionary relationship than with bacteria.
How can RNA polymerase be inhibited?
Rifampicin - inhibits transcription by interacting with catalytic Beta subunit of RNA polymerase, preventing DNA transcription.
ACtinomycin - Binds next to gene promoter region, inhibiting the binding of the RNA holoenzyme (RNA pol and sigma factor).
