Monocistronic vs Polycistronic Genes

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Transcription in Organelles
Types of RNA polymerases Composition of RNA
polymerases Sensitivity to drugs Similarities/di
fferences between mitochondria and
chloroplast RNA polymerases Some evolutionary
perspectives
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Transcription in Organelles
What about transcription of these highly
evolved/derived genomes?
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?Mitochondrion from Bean root tip
Mito. from transfer cell?
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Mitochondrial RNA Polymerase A phage-like RNAP

• Core enzyme is a 1-subunit, phage-like enzyme.
• Also has a specificity factor that is needed to
  initiate RNA synthesis at promoter.
• Similar core enzyme in animal, fungal and plant
  mitochondria, but different specificity factors.
• Specificity factor in yeast resembles the
  bacterial s factor core regions, particularly
  region 2 that binds the -10 region of promoter.
• Promoter is usually a 9-10 bp sequence.
• Genes encoded in the nucleus.

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Evolutionary questions about the Mitochondrial
RNA Polymerase

• How and when was the phage RNAP gene acquired?
• What happened to the bacterial RNAP genes in the
  original endosymbiont?

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Look at earlier eukaryotes

• In Pylaiella, a eukaryotic brown alga, a
  phage-like polymerase gene is in the
  mitochondrial genome.
• Suggests that the nuclear-encoded, phage-like
  mitochondrial RNAP came from the endosymbiont.

a.k.a. Mung
7

• In Reclinomonas, another primitive eukaryote,
  bacterial RNAP (rpo)-like genes are found in the
  mitochondrial DNA (pseudogenes?).
• Suggests that the endosymbiont also had rpo
  genes.

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Transcription in Chloroplast
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Isolated Spinach chloroplast
envelope
thylakoid membrane
stroma
From Hoober
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Chloroplast RNA polymerases (RNAPs)

• Two different RNAPs in vascular plant
  chloroplasts
• Bacterial-like polymerase (also called PEP,
  plastid-encoded polymerase)
• Phage-like or NEP (nuclear-encoded polymerase)
  polymerase

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Chloroplast Bacterial-like RNAP

• composed of Core Sigma factor
• Core 4 subunits, a2 b b'
• a is encoded by the rpoA gene
• b is encoded by the rpoB gene (also sometimes
  split)
• b' is encoded by the rpoC1 and rpoC2 genes
• Sigma factor needed to initiate transcription at
  the promoter (recognizes -10, -35 promtoers)
• Nuclear encoded, family of 6 genes in
  Arabidopsis
• Inhibited by rifampicin

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Chloroplast Phage-like (NEP) polymerase

• Gene is similar to the 1-subunit phage (e.g., T7
  phage) RNA polymerases
• Nuclear gene(s)
• Enzyme insensitive to rifampicin
• Promoter is a single region of 7-10 bp
  (different from -10,-35 promoters)
• Specificity factor not yet identified

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Some chloroplast genes have promoters for both
the PEP and NEP RNAPs..
Fig. 6.31 in Buchanan et al.
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Why is chloroplast transcription much more
complex than mitochondrial transcription?
Chloroplasts are larger, more complex organelles
than mitochondria. They also differentiate.
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Chloroplasts are only the main form of a class of
Organelles Plastids

• 1. Proplastids - precursor of all plastids, found
  in meristems
• 2. Etioplasts - form in shoots of dark-grown
  plants, distinctive internal structure
• 3. Chloroplasts - in all green tissues
• 4. Amyloplasts - prominent in roots, store
  starch, colorless
• 5. Chromoplasts - in mature fruit, lots of
  carotenoids, little chlorophyll

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From U. Wisconsin Botany Dept.
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Plastid types develop from proplastids Shoots
light -----gt proplastids lt----gt
etioplasts lt----gt chloroplasts
chromoplasts Roots proplastids lt-------gt
amyloplasts
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Complex suite of RNAPs provides for developmental
regulation.

• NEP more important in proplastids, when there is
  not much plastid gene expression (which is needed
  to make the rpo genes).
• PEP becomes very important in chloroplasts.
• There is also regulation of the PEP polymerase
  via the sigma factors
• - selective transcription by certain sigma
  factors
• - greater phosphorylation of major sigma
  factors in chloroplasts compared to etioplasts

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Monocistronic vs Polycistronic Transcription

• Prokaryotes Both
• Eukaryotes
• Nucleus Monocistronic (polycistronic rare)
• Mitochondria
• Mammals polycistronic (2 promoters) pre-RNAs
  processed into smaller species
• Other Both
• Chloroplast - Both