PCB6528 Spring 2014 - Exam 2 – Chase
Name ____________________________
The exam was written based on 100 points. Dr. Cline will weight its contribution to your final grade
based on the fact that the exam covers material presented in 6 classes.
Important – please don’t write a lot! Questions can be successfully answered in 1-3 short sentences.
Use the space provided and please don’t write on the back of any pages!
1 [35 pt] In our discussion paper, Stegmann et al. (Proc. Natl. Acad. Sci. USA 109:2434) used four
different transgenes in their work to look for chloroplast capture in the context of tissue grafting:
• Plastid rrn promoter driving gfp
• Plastid psbA promoter driving aadA (spectinomycin/streptomycin resistance)
• Nuclear nos promoter driving nptII (kanamycin resistance)
• Viral 35S promoter driving yfp
1-a) Describe what is meant by chloroplast capture
1-b) What was the specific function of the antibiotic resistance genes in this study?
1-c) What was the specific function of the gfp and yfp genes in this study?
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PCB6528 Spring 2014 - Exam 2 – Chase
Name ____________________________
1-d) What behaviors of plant mitochondria and plant mitochondrial genomes might be expected to
create mitochondrial genetic exchange in the chloroplast capture plants, even in the absence of a
mitochondrial selection agent?
1-e) Successful chloroplast capture through tissue grafts was mentioned as one way to overcome
barriers to chloroplast transformation in some plant species. In theory one could transform the
desired constructs into tobacco plastids and move them into another species. Briefly discuss two
reasons why this strategy might be limited to species that are closely related.
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PCB6528 Spring 2014 - Exam 2 – Chase
Name ____________________________
2 (35 pt)
2-a) Plant organelles have a unique RNA editing process that is essential to organelle gene
expression. In tobacco plastids, for example, the initiation codon of the psbL gene must be created
by RNA editing. The genomic sequence at the 5' end of the tobacco gene begins with ACG. In the
table below, fill in the nucleotide sequences that correspond to this codon for each different
molecule.
molecule
psbL initiation codon
(all in the 5’ to 3’ orientation on
the coding strand)
tobacco psbL genomic DNA
5' ACG 3'
tobacco psbL un-edited RNA
tobacco psbL cDNA corresponding to the un-edited RNA
tobacco psbL edited RNA
tobacco psbL cDNA corresponding to the edited RNA
2 -b) Briefly describe one line of evidence that plastid RNA editing sites are defined by specific
nucleotide sequences “in cis” (nearby) to the edited nucleotide.
2-c) List two protein families that have been implicated as trans-acting factors in plant organelle RNA
editing.
2-d) Discuss the evidence that plastid and plant mitochondrial RNA editing likely occur by the same
mechanism.
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PCB6528 Spring 2014 - Exam 2 – Chase
3 (30 pt) The scheme for
photosynthetic electron
transfer is shown in the
diagram to the right. If
light conditions are
altered, plastid
transcription will be
adjusted so that the two
photosystems can
maintain the balance of
electron flow through the
pathway.
Name ____________________________
PSI
PSII
PET
FD
3-a) If light is changed from a wavelength favoring photosystem II (PSII) activity (light II) to a
wavelength favoring photosystem I (PSI) activity (light I) would the ratio of transcripts for the
PSII/PSI core subunits (psbA / psaA) be expected to increase or decrease? Explain your reasoning.
3-b) Briefly describe the experimental observations supporting the hypothesis that the redox status
of plastoquinone (PQ) is central to the signaling that adjusts plastid transcription to achieve redox
balance between the two photosystems.
3-c) Define retrograde regulation and explain why it this process would also be necessary to
maintain optimal redox balance in the plastid.
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