Which bacterial features are present in plastidic DNA? Give three properties.
• Circular double-stranded DNA (like bacterial chromosomes)
• Operons / polycistronic mRNA transcripts
• Prokaryotic translation machinery: 70S ribosomes, Shine-Dalgarno sequences, initiation with N-formylmethionine
Which feature(s) concerning overall genomic organisation are typical for plastids? (3)
• Inverted repeat (IR) regions in the genome
• High copy number of plastid genomes per plastid
• Compact genome: very little non-coding DNA, few introns
3. Expression of most plastid-encoded genes is under translational control. Which part of the mRNA is decisive for this regulation?
The 5′ UTR (untranslated region), especially the Shine-Dalgarno sequence and adjacent secondary structures
Briefly describe the pathway to a mature PsbO protein in vascular plants (nuclear-encoded, located in the thylakoid lumen), starting from the location of translation. Describe changes to the polypeptide chain and label energy-dependent steps.
Translation:
• Occurs on free ribosomes in the cytosol
• Nascent chain contains an N-terminal bipartite targeting peptide:
• Chloroplast transit peptide (cTP)
• Thylakoid transfer domain (signal peptide)
Transport through the envelope:
• Import via TOC/TIC complexes (Outer/Inner Chloroplast membrane)
• Energy: GTP at TOC receptors; ATP in the stroma (Hsp70/Hsp90 chaperones)
• In the stroma, the cTP is cleaved by Stromal Processing Peptidase (SPP)
• Protein now has only the thylakoid signal peptide
Transport through the thylakoid membrane:
• PsbO uses the Sec pathway (for unfolded proteins)
• Translocation via SecA/SecYE
• Energy: ATP (SecA ATPase) and/or ΔpH (depending on pathway)
• In the lumen, the thylakoid signal peptide is cleaved by Thylakoid Processing Peptidase (TPP)
• Mature PsbO folds and integrates into the oxygen-evolving complex (OEC) of PSII
Polypeptide changes summary:
1. Synthesized as precursor with cTP + thylakoid signal
2. cTP removed in stroma → intermediate form
3. Thylakoid signal removed in lumen → mature PsbO
5. In organisms with secondary endosymbiosis, transport is more complex due to additional envelope membranes. Which protein complexes carry out the additional transport? (2)
• Outermost membrane (derived from ER): Sec61 translocon (classical ER translocation with signal peptide)
• Second envelope membrane (periplastidial membrane): SELMA (Symbiont-specific ERAD-like Machinery) Wikipedia:Nucleomorph
What is the regulatory signal from the nucleus to the organelles called?
Anterograde signaling (nucleus → plastid/mitochondrion)
Give one example of such a signa (nucleus to plastid)l:
• Mg-Protoporphyrin IX (plastid-to-nucleus retrograde signal, but often discussed in anterograde context)
• Or GUN1-mediated signal (Genome Uncoupled pathway)
• Common example: 3′-phosphoadenosine-5′-phosphate (PAP) under stress
8. Change in light quality leads to differences in excitation of PSI and PSII. What is the process called that regulates relative antenna size to balance excitation energy input?
State transitions (State 1 ↔ State 2)
9. Which method can be used to demonstrate differences in antenna size and relative excitation energy transfer?
• 77 K fluorescence spectroscopy (separates PSI and PSII emission peaks)
• Or PAM chlorophyll fluorescence measurements
10. Which post-translational modification is needed for this process?
Phosphorylation (of LHCII)
Which proteins are mainly modified?
LHCII proteins, especially Lhcb1 and Lhcb2
How is the process activated?
• Over-excitation of PSII → reduced plastoquinone pool → activates STN7/Stt7 kinase
• Kinase phosphorylates LHCII → LHCII migrates to PSI → State 1
13/14. Plants use photoreceptors. Which chromophore is responsible for signal perception in:
• Phototropin
• Cryptochrome
• Phototropin: FMN (Flavin Mononucleotide)
• Cryptochrome: FAD (Flavin Adenine Dinucleotide)
15. Signal transduction from cryptochromes 1 and 2 in plants includes binding of two further proteins usually needed for degradation of transcription factors. Which are they?
• CUL4 (Cullin 4) and DDB1 (Damaged DNA Binding protein 1)
• Together they form the CUL4–DDB1 E3 ubiquitin ligase complex
16. How is the thylakoid morphology in the chloroplasts of red algae, green algae, and brown algae? (
17. Which pigments are found in the light-harvesting complexes of vascular plants and diatoms? Mention 3 pigments each:
What is a nucleomorph?
• A remnant eukaryotic nucleus located between the inner and outer envelope membranes of plastids in some algae with secondary endosymbiosis
• Found in cryptomonads and chlorarachniophytes; proves secondary endosymbiotic origin Wikipedia:Nucleomorph
How is DNA transferred into diatoms?
Via horizontal gene transfer and endosymbiotic gene transfer from the plastid/nucleomorph to the host nucleus during evolution
Which genetic elements do episomes need minimally?
1. Origin of replication (ori)
2. Selectable marker (e.g., antibiotic resistance)
3. Promoter (for transcription)
4. Terminator
5. Multiple cloning site (MCS) / insertion site
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