Organelle Genomes
PLANT GENETICS and MOLECULAR BIOLOGY
Department of Cell Biology & Molecular Genetics
I. Non-Mendelian Inheritance
-maternal inheritence- depends on parent
II. Origin- Endosymbiotic Theory 1-2 billion yrs ago
mitochondria- purple photosynthetic bacteria
plastid-chloroplast- cyanobacteria
similarities:
circular DNA
DNA lacks histones
ribosomes similar 70S (50 and 30)
inhibited by similar antibiotics (e.g chloramphenicol, streptomycin)
different GC content ( 36%) than nuclear (64%)
Intermediates in Endosymbiosis-secondary endosymbiosis (only with plastid)
-in certain algae-chlorarachinophytes ameboid algae
see two sets of rDNA-chloroplast has 4 membranes
genome outside of chloroplast is nucleomorph
others- another algae which swallowed red algae
-Phylum Apicomplexa
-plasmodium (malarial parasite), toxiplasma
-plastid but non-photosynthetic- secondary plastid from green algae
III. Chloroplast Genome
-22-900 copies per plastid
-associated in aggregate called nucleoids
-Overall organization
-120-400 kB in size
-large inverted repeat (IR)
-3 classes, based upon presence of inverted repeat
-Genes
-100 protein coding genes, rDNA, tRNA
-loss of 90% of cyanobacteria genes (3,500 genes)
genes in chloroplast
a. ~40 Photosynthetic genes
b. ~ 50 needed for replication, transcrition, translation
rRNA, some tRNA, polymerase
c. unknown
-variable e.g. rice & maize have only 1
-red algae 30 others- aa synthesis, protein transport
IV. Mitochondrial Genome
sizes 208(watermelon) -2,500 kB (muskmelon)
-Comparison with animals
-human- simplest 16,569 bp , compact
-tightly packed 2 rDNA
22tRNA
5 membrane proteins
8 unassigned
-2 RNA transcripts make copies of each half
-tRNAs flank mRNAs- all cut from two transcripts during
transcription
-yeast
-5x as much DNA10 open reading frames and 6 membrane proteins
genes separated by AT rich stretches
-plants- 5x larger than yeast
- genes in mt- more genes than animals (Bryophyte 94 ORFs)
rRNA 26S, 18S, 5S (plants only)
ribosomal proteins- one in some plant mt
tRNA- full complement
protein-coding
some of inner membrane complexe’s subunits
-replication, transcription, translation
cytochrome C
ATPase
(mammals- subunit 9 in nucleus, plants,
yeast in mt)
apocytochrome B of bc1 complex
-Organization- tripartite(unlike animals- single circle)
-master circle and smaller subgenomic circles
-master produces two genomics through recombination
Why not a mess of fragments?
Recombination gives produces variability, mutation rate 10x lower than nucleus
-e.g cytoplasmic male sterility in maize
-mt gene- tURF13 114 codons
88 aa similar to rrn26 (ribosomal RNA)
8 unknown
18 more similar to rrn26
flanking 5 ‘ region(5kB) atp26 gene
-in anthers, some anther-specific gene interacts- makes mt leaky so dies
-T-toxin in Cochliobolus maydis does same
-RNA editing:
-discrepancies between DNA sequence and protein sequence in wheat
-originally thought to be deviation in code CGG arg->trp
-sequenced cDNA found cGG changed to UGG
-C ->> U, sometimes U->> C
-depends on gene, no consensus
-other editing
-mt and cht already unusual in cutting RNA to give multiple subtranscripts of
mRNA as well as rRNA and tRNA (latter two normal)
- cis and trans splicing, often combined
Promiscuous genes- found in different compartments
-Cyanophora- rbcS still in chloroplast, next to rbcL
-chloroplast sequences in mt DNA of maize
-Many of these look like reverse-transcribed copies
-introns also hopping around- self splicing type I introns found to move, also see
movement from fungi into plants
Links
Organelle genome database
Example of secondary
endosymbiosis
Chloroplast
genome organization- PLSC731 NDSU
Mitochondrial
genome organization- PLSC731 NDSU
Summary
of Mitochondrial Genome
RNA
editing- PLSC731 NDSU
Last revised: March 17 1998- Straney