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mobile introns

newsmgr at merrimack.edu newsmgr at merrimack.edu
Tue Nov 11 22:54:15 EST 1997

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Subject: Re: mobile introns
Message-ID: <3468B0B3.76E4 at evol5.mbl.edu>
From: "Andrew J. Roger" <roger at evol5.mbl.edu>
Date: Tue, 11 Nov 1997 15:23:30 -0400
Reply-To: roger at evol5.mbl.edu
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cok at mole.bio.cam.ac.uk wrote:
> I'm currently teaching some stuff on genome organisation, which includes
> such things as introns early/late, mobile DNA (including self-splicing
> introns).
> A phenomemon that I can't seem to rationalise is the (relative)
> confinement of mobile self-splicing introns to organelles. I appreciate
> that:
> - they may have come into eukaryotes with organelles; BUT this hasn't
> stopped loads of other organelle genes moving to the nucleus.
> - they may have evolved into spliceosomal introns in the nucleus, as their
> host organism would develop an interest in making damned sure that it
> retained the machinery to splice them effectively out of coding regions.
> BUT I don't see any reason why mobile self-splicing introns couldn't be
> maintained in the longer term in nuclear genomes just as well as they have
> been in many mitochondrial genomes.

You should be a little careful to not lump different introns into the
same class. Mobile self-splicing introns are actually a broad category
that includes both group I and group II introns.  Your point only
applies to group II introns since none have been found intact and
working in nuclear genomes.  Group I introns have been found in many
eukaryotic nuclear ribosomal RNA genes-- for instance they are found in
some chlorophytes (Daneliella), a variety of Naegleria species,
Tetrahymena, some fungi and several other protist groups. Just do a
BLAST search with a group I intron in the database and you'll find tons
of examples of nuclear ribosomal RNA genes with group I introns in them
(there is probably a recent review paper that summarizes the
phylogenetic distribution of these introns).  The question about these
introns is why they are never found in other genes but for ribosomal RNA
(I'm not sure if this is true anymore, but last time I checked it was)

Why group II introns are not in nuclear genes is truly a mystery. 
Fragments of group II introns have been found in the nucleus (see Knoop
and Brennicke,
1991, Curr. Genetics, 20:423-425 for example)...but none intact.  I
favour the idea that these introns may be spliceable by the spliceosome
and therefore immediately lose the need to self splice.  If they were
able to spread faster than they decayed due to neutral drift after
complementation by the spliceosome then we ought to see them
around.....but we don't.  This could be the result of the presence of a
debranching enzyme which is known to be present in the nucleus that
prevents some of the steps in intron spread (for instance reverse
splicing of the lariat intron into the double stranded DNA).

> - long-term maintenance of mobile elements in a genome usually requires
> sex, so that elements can spread to unpopulated genomes. BUT this should
> be even less of a barrier for nuclear mobile introns than for organelle
> ones.

This is true....and I doubt that it is lack of sex which is preventing
the spread of group II introns in the nucleus..

to summarize I think it may be either that:

1) group II's do go to the nucleus but quickly decay into spliceosomal
2) there is some selective reason why highly structured introns cannot
survive in nuclear protein coding genes that are transcribed by RNA

> Thoughts on the back of an envelope, please, to
> Cahir O'Kane
> Dept of Genetics, University of Cambridge, Downing Street,
> Cambridge CB2 3EH, UK
> c.okane at gen.cam.ac.uk
> --
> Cahir O'Kane, Dept of Genetics, Univ. of Cambridge, UK
> cok at mole.bio.cam.ac.uk

Andrew J. Roger

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