> In some cases (and maybe in all) the differences in genetic code are
> introduced by mRNA processing. This means that the chromosomal DNA
> of the organell codes with the usual code but then the mRNA is modified
> after transcription. This is maybe done because some organelles
> have a reduced set of tRNA and therefore are not adapted to the
> redundancy of the genetic code.
All interesting stuff, and great a source of potential confusion.
Here's some tables from Mol.Cell Biology by Darnell, Lodish & Baltimore.
-----------------------------------------------------------------------
CODON Normal Assignment Mitochondrial Assignment in
Humans Yeast Neurospora
UGA Stop Trp Trp Trp
CUU |
CUC |
CUA > Leu Leu Thr Leu
CUG |
AUA Ile Met Met Ile
AGA
AGG Arg Stop Arg Arg
---------------------------------------------------------------------
======================================================================
An inventory of genes in yeast mitochondrial DNA:-
Mitochondrial Component Mitochondrial Gene Product
====================== ==========================
Large ribosomal subunit 21S rRNA
(21S rRNA, 38 proteins)
Small ribosomal subunit 15S rRNA
(15S rRNA, 33 proteins) var-1, a ribosome associated protein
tRNAs (~30) All of them
Cytochrome c oxidase Cytochrome c oxidase
(9 subunits) subunits I, II, & III
Cytochrome b/c1 complex Apocytochrome b
(7 subunits)
ATPase complex Subunit 9*, subunit 8
(10subunits) subunit 6
----------------------------------------------------------------------
*encoded in nuclear DNA and made in the cytoplasm in Neurospora,
Aspergillus, and humans
=====================================================================
SOME MITOCHONDRIAL PROTEINS SYNTHESIZED IN THE CYTOPLASM
Intra-mitochondrial location Polypeptide*
============================ ===========
Matrix F1-ATPase, alpha, beta, gamma, subunits
Carbamoyl phosphate synthetase
Mn2+ superoxide dismutase
RNA polymerase
Ribosomal proteins
Citrate synthetase & other citric acid
cycle enzymes
Ornithine transcarbamoylase
Inner Membrane Cytochrome c1
Subunit V of cytochrome b/c1 complex
ADP-ATP carrier
Cytochrome c oxidase,
subunits IV, V, VI, VII
Proteolipid of F0-ATPase complex
Intermembrane space Cytochrome c
Cytochrome c oxidase
Cytochrome b2
Outer membrane Porin
----------------------------------------------------------------------
*most proteins except ADP-ATP carrier, cytochrome c, and porin are
fabricated as longer precursors.
======================================================================
This all shows that there is a considerable traffic of gene products into
the mitochondrion.
BUT remembering the genetic code question, I always thought that the
*embodiment* of the genetic code is the set of amino-acyl-tRNA synthetases
which effectively "carry" or "perform" the correspondence between an
anti-codon on the tRNA and the corresponding amino acid to which it gets
joined.
The above tables tell us nothing about mitochondrial tRNA synthetases,
but I would have thought that they are encoded in the mtDNA.
Anybody know anything about chloroplast genomes?
Rgds.,
Alan Mills at Venus Internet Ltd, 24 Denmark Street, London WC2H 8NJ, UK
email:alan@venus.co.uk web:http://www.venus.co.uk
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