Bacterial organelles and substructures- lecture 25 Flashcards
How are magnetotaxis arranged in the bacteria?
They are lined up in the center of the cell.
General organelle structure
- proteinaceous or membranous barriers keep it separate
- performs a specialized function at a specific sub cellular location
- sometimes factor must be partitioned from the rest of the cell to protect their function
- must be synthesized or expanded, must be segregated to daughter cells
What is the general structure of the the bacterial nucleoid?
- highly compacted region on center of cell where DNA resides
- Not surrounded by a membrane or protein shell
- > accessible to medium sized proteins
- Not permeable to large components like ribosomes
- serves an essential function in all bacteria- organizing in DNA in a compact yet accessible manner, and segregating it to daughter cells before cell division
General bacterial chromosome structures
- most bacteria have a single, circular chromosome
- they are haploid (1n)- 1 instance of each gene
- before replication, we refer to the single genome as 1c, after replication, there are two identical copies
Different bacteria and their ori/ter sites
Bacillus Subtillus(sporulating): ter sites facing inward, origin sites facing out.
- origin at poles
C. Crescentus: Ter site at one end, originally site at other
- origin replicated and moves to other pole
E.coli slow growth: Ori and term in center
E.Coli fast growth: occurs in rich media, high temp, multiform replication
How do bacterial chromosomes replicate?
Replication begins at ori site and ends at term.
Replication can begin before its finished=> multifork replication
Multiple replication forks meet in the middle at the terminus
Why is multiform replication beneficial
A cell can get a head start to make more DNA if it gets a head start-> can make more offspring-> complete cell cycle quicker
How can we visualize DNA loci in bacteria?
LacI is the DNA binding protein-> binds at Lac O
Determine where cell genomic loci localize within a cell through fluorescent reporter operator system (FROS)
- uses site specific DNA binding proteins fused to fluorescent protein
- place array of binding sites near locus you want to investigate=> fluorescence coalesces into a focus at that site
- different DNA binding proteins/sites labeled with different colors can probe multiple sites
What is transertion?
Genes encoding membrane proteins are transcribed, translated, and the polypeptide is inserted into the membrane at the same time
=> loops of DNA have to access the membrane of a bacterial cell
How do bacterial cells increase nucleoid compaction>
The DNA supercoils
How do transcription and translation affect DNA compaction?
Transcription causes DNA to condense, potentially through formation of supercoils or multiple RNAPs interacting with each other
Translation causes DNA to expand, likely because the ribosomes lie outside the nucleoid and the DNA must travel to the membrane for transertion
What do Nuceloid associated proteins do?
Bind to DNA and help it compact/supercoil
more are found where more are needed, origin of replication
What is the function of parABS
ParB is a DNA binding protein that binds to origin proximal sites called parS sites
-> parB proteins spread out along the DNA near the parS site, condensing it and recurring SMC proteins
What is the SMC complex?
- encircles double helix
- stands for Structural maintenance of Chromosomes, is a ring shaped protein complex that is thought to encircle the DNA double helix
- analogous proteins are found in all domains of life (in eukaryotes-cohesin and condensin)
How to segregate replicated chromosomes
SMC replication rings are loaded at origin and slide down DNA, surpassing protein complex and transcription/translational things
Purpose: Compact and resolve newly replicated chromosomes
ParA and parB push/pull chromosomes apart
What proteins/actions are involved in segregating the replicated chromosomes?
To separate the strands, they need to be unlined. When they are replicated they either make a continuous chromosome dimer or a two-chromosome linked chromosome concatenation.
Topoisomerase(gyros and topoIV) cut and twist DNA to relax it, but they don’t always complete the job and the chromosomes end up linked
Homologous recombination between the chromosomes van turn them into a single chromosome.
How and when are linked chromosomes resolved?
Linked chromosomes are separated by the closing of the septum.
FtsK is a DNA-pumping machine that pumps DNA out towards terminus, it activated TopoIV to resolve concatenated DNA then FtsK activates recombinases XerC and XerD to resolve dimers.
Organelles that store nutrients.
Iron storing granules
membrane bound iron storage compartments
assembly controlled by fez genes, expression of fez in E.coli led to ferrosome formation
ferrosome membrane contains Iro-pumping channel called FezB that helps accumulate Fe2+
organelles functioning in locomotion/taxis
Gas vesicles and magnetosomes
magnetosomes have magnetite or reignite that form linear chains, enable magnetotaxis-locomotion oriented along the earths magnetic field
-magnetite is deposited inside the magnetosome, expand as mineral gets bigger
mam genes are responsible for magnetosome biosynthesis, 35 mam genes are sufficient to form magnetosomes
Actin homolog mamK forms a filament that organizes magentosomes into a chain, mamK is required to evenly segregate magentoseomes into daughter cells
organelles that function in metabolism
Carboxysomes have protein shells that contain carbon fixation machines and trap CO2
Annamoxosomes are membrane bound organelles where anaerobic oxidation of ammonia takes place
Thylakoids are membranous intercellular structures where light driven photosynthesis reactions occur
Concentrating metabolic enzymes and metabolites enhances the efficiency of these processes
Caboxysome
Surrounded by a protein shell impermeable to CO2 and O2, but bicarb can be diffused in and converted to CO2 by carbonic anhydrase
-> We want O2 out because ut competitively binds to Rubisco
ParA like protein McdA oscillates along nucleoid, carboxysomes bind McdA for even distribution
Organelles that serve to isolate/protect their contents
- Jumbophage nucleus: large genomes, very complex, form protective shell
During infection, phage genome does not have to be segregated because the cell will lyse before division
Proteinaceous shell encapsulates the phage DNA, phage RNA is transcribed inside
=> separate transcription and translation
Allows to evade DNA targeting defenses like restriction endonuclease and CRISPR
