Re: Ribosomes

Dr Alan Mills (
Tue, 26 Mar 1996 13:16:32 +0000 (GMT)

On Tue, 26 Mar 1996, Peter Slickers wrote:

> 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

CUA > Leu Leu Thr Leu

AUA Ile Met Met Ile

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


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

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?


Alan Mills at Venus Internet Ltd, 24 Denmark Street, London WC2H 8NJ, UK web:
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