Download Initiator vs. elongator Met tRNA

Initiator vs. elongator Met tRNA
pseudouridine
The two major differences are:
1) a missing base pair in the acceptor arm of the fMet-tRNA
2) 3 GC pairs in the anticodon stem of fMet-tRNA
http://en.wikipedia.org
Rasmussen, L. C., Laursen, B. S., Mortensen, K. K. and Sperling-Petersen, H. U. 2009. Initiator tRNAs in Bacteria and Eukaryotes. eLS.
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Apparently, one tRNA synthetase is used to attach methionine to both tRNAs.
The formyl group is added later by the enzyme methionyl-tRNA formyltransferase
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The mitochondrial genome
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Human mitochondrial
genome:
13 proteins (all used by the
mitochondria to generate ATP)
2 rRNA (ribosomal RNA)
22 tRNA (one for each a.a. plus
extra one for Leucine and
Serine)
Of the 37 genes on the mtDNA, 28
of them are read off one strand
(heavy strand) and 9 are read off
the other (light strand).
Lang et al. (2003) "Mitochondrial Genome Evolution." In: Encyclopedia of Biological Chemistry, Vol.3 pp.703-707
http://en.wikipedia.org/wiki/Human_mitochondrial_genetics
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As we saw earlier mtDNA is a relatively small (16569 bp) circular piece of DNA that resides in the
mitochondria. Each cell has many hundreds of copies.
Since it is in the mitochondria, transcription and translation of these genes must be somewhat different than
it is for genes encoded in the nuclear genome.
On the other hand, there are clearly not enough genes in the mtDNA to do all of the work. That is, there are
no mRNA modifying genes, DNA polymerases, RNA polymerases, tRNA synthetases, or ribosomal proteins.
All of these must be sent to the mitochondria after being synthesized by ribosomes in the cytoplasm.
Interestingly, it would seem like the cell is not able to ‘target’ tRNAs or rRNAs to the mitochondria. For this
reason, these components must be made in place.
How can just 22 tRNAs recognize the 61 codons (64 - 3 stop codons)?
RNA processing in the mitochondria
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RNA polymerase
mTERF1 stops
transcription from
HSP2
Human mtDNA only has 3
promoters, 2 for the heavy chain
(HSP1 and HSP2) and 1 for the
light chain.
This means that all of the genes
(including mRNA, tRNA, and
rRNA) get synthesized as one
long strand.
Pearce, S., et al., Mitochondrial diseases: Translation matters, Mol. Cell. Neurosci. (2012)
Rackham et al. The human mitochondrial transcriptome and the RNA-binding proteins that regulate its expression. WIREs RNA 2012, 3: 675-695
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Since the mtDNA is transcribed as one long RNA molecule, it needs to be cut into smaller pieces that
represent the gene products.
The tRNA sections of the RNA will fold up, even in the context of the longer chain
RNAse enzymes recognize the folded tRNA and cut the RNA at the 5’ and 3’ ends to release the tRNAs. As
you can see from the genome, the tRNAs occur between the other coding sequences, so by releasing the
tRNAs, the other sequences are also released
Further processing gives the final tRNA, rRNA, and mRNA molecules
There are no introns in human mitochondrial genes
There is no 5‘ cap on mitochondrial mRNA, though there is a poly A tail.
For translation, ribosomes are assembled in the mitochondria from the mitochondrial rRNA plus additional
proteins encoded in the nuclear genome.
Two models of how mtDNA is replicated
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The displacement replication mechanism
seems to have more evidence to support it.
Timothy A. Brown, Ciro Cecconi, Ariana N. Tkachuk, et al. Genes Dev. 2005 19: 2466-2476
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mtDNA replicates in a way that avoids the ‘lagging strand problem”
Essentially, an RNA primer is extended by DNA polymerase on just one strand. This displaces the other
strand, which spends time as a ssDNA coated with ssDNA-binding proteins.
When a certain position on the displaced strand becomes exposed, another RNA primer can bind, and
initiate DNA polymerization going the other direction.
In the atomic force microscopy (AFM) images shown on the right, the bright spots are the ssDNA binding
proteins coating the displaced DNA (represented in red in the cartoon). Blue lines are the parent strands,
aqua lines are newly synthezised H-strand DNA, and green lines are newly synthesized L-strand DNA