In article <4piev4$es5 at elaine13.Stanford.EDU>, ladasky at leland.Stanford.EDU
(John Ladasky) wrote:
>Introns are not found in bacterial DNA...
Not quite right. You find both types of the so-called autocatalytic
introns (Group I and Group II) in eubacteria like cyanobacteria and even
Escherichia coli, although they are admittedly very rare.
> I am not aware of any grand consensus about
> the origin, original function...
No, definitely no consensus. What I consider a good working hypothesis is
that autocatalytic introns of group I and group II arose in bacteria
rather early by chance and may really be considered as selfish sequences.
The endosymbiontic bacterial predecessors of mitochondria and chloroplasts
may have imported them into the eukaryotic cell. (These introns are
present in mitochondria and chloroplasts!!) Group II introns may have
invaded the nucleus and have evolved into what you today find as typical
eukaryotic spliceosomal introns.
Now these latter may indeed have gained later certain functions for
example having enhancer functions or by allowing for different products to
be made from a single gene via alternative splicing. Exon shuffling is
certainly another idea for ascribing function to introns but not so
popular any more.
Volker Knoop (volker.knoop at biologie.uni-ulm.de)
Allgemeine Botanik, Universitaet Ulm, D-89069 Ulm, Germany
phone +49-731-502-2615 fax -2626