Re: Ribosomes

Peter Slickers (
Tue, 26 Mar 1996 11:12:54 +0200 (MDT)

John Walshaw wrote:
> In textbooks it says something to the effect that mitochondria and
> chloroplasts 'contain complete genetic systems', but that the proteins
> which mediate these systems are mostly encoded by the nuclear genome.
> By the way, I seemed to remember from my undergraduate cell biology (some
> time ago admittedly) that some of the rRNA molecules of cytoplasmic
> ribosomes are encoded by mitochondrial DNA, and mentioned this- but looks
> like my memory failed me here, as this is wrong.
I have never heared of rRNA tranfer from organelles into the cytoplasma,

> But is it correct to assume that the products of these mitochondrial genes
> arent enough to make a mitochondrial ribosome? I.e., the organelles must
> import additional rRNA molecules coded by the nuclear genome. If so, are these
> imported rRNAs the same as those included in cytoplasmic ribosomes, or
> are they specific 'mitochondrial ribosome rRNAs' encoded by the nuclear
> genome?
Most probably, the nuclear genes which code for the proteins which are
exported into the organelles have jumped from the organelles into
the nucleus in an early stage of the organell's evolution. So, this genes
are prokaryotic although they are now localized in the nucleus. One hind
for this is the "strange" way by which proteins are imported into
mitochondria by use of two leader peptide sequences, a eukaryontic and a

> Another thing- the implications of the different genetic codes found in
> 1)chloroplasts 2)mitochondria and 3)both prokaryotes, and eukaryote
> nuclear genomes.
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.

> What are the implications for the endosymbiont theory for the origin of
> these organelles (i.e. one prokaryote ingesting another prokaryote). I
> can think of a couple of points, but can anyone enlighten us?
Prokaryotes are devided into archaebacteria and eubacteria. On a
evolutionary scale the archaebacteria are as vary apart from the
eubacteria as the eubacteria from the eukaryonts. The organelles
(chloroplasts and mitochondria) are close related to the eubacteria
whereas the eukaryotic cell itself has evolved from the
branch of the archaebacteria.


Peter Slickers                        
Institut fuer Molekulare Biotechnologie
Beutenbergstrasse 11, Jena                      Tel.: +49-3641-65-6202
Postfach 10 08 13, D-07708 Jena, Germany        Fax:  +49-3641-65-6210