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Will Fischer wfischer at sunflower.bio.indiana.edu
Thu Jun 13 17:43:46 EST 1996

John Ladasky (ladasky at leland.Stanford.EDU) wrote:
:                                                    	...... There are
: conserved intron sequences that cause the primary RNA transcript to fold
: into a defined structure that includes the ends of the intron.  This "ribo-
: zyme" construct then splices itself out of the primary RNA.  

This is only correct for the self-splicing group I and group II introns
found in organelle (and certain eubacterial) genomes.  The introns
familiar to more people are spliceosomal introns (the only type yet
found in eukaryotic nuclear genomes).  These are spliced by an
assemblage (the spliceosome) of small ribonucleoproteins (snRNPs) which
bind to consensus sequences at both splice sites.  The spliceosome _is_
thought to be a ribozyme.
                                            [ SOME, not all ]  Introns also
: carry "enhancer" sequences at the DNA level.  These sequences are believed
: to target transcription-enhancing proteins to the vicinity of certain
: genes.
: 	There are a lot of theories about the formation of introns and
: their utility.  Introns are not found in bacterial DNA, but the debate
: rages on about whether they existed at one point and then were eliminated.
: So the evolutionary picture is unclear.  

One school of thought maintains that the spliceosomal introns seen in
modern eukaryotic genes are descended from introns (in the same
positions) in the genes of a common ancestor of all living things.
These primoridal proto-introns are posited to have been involved in
early gene assembly (via exon-shuffling), and to have been subsequently
lost in the eubacterial and archaebacterial lineages.

An opposing school of thought suggests that these same eukaryotic
spliceosomal introns result from insertions of novel elements (probably
derived from group II introns) that arose in the eukaryotic lineage

Look up W. Gilbert for introns-early and exon-shuffling refs, (e.g.
Gilbert & Glynias (1993) Gene 135:137-144,  Long et al. (1995) PNAS
USA. 92, 12495-12499).

For introns-late, try
J.D. Palmer (e.g. Logsdon et al., PNAS U.S.A. 92:8507-8511,
Palmer & Logsdon 1991 Curr. Opin. Genet. Dev. 1:470-477),
Cavalier-Smith (e.g. (1991) Trends Genet. 7:145-148.)
Doolittle & Stolzfus 1993 Nature 361:403.

Huge topic (arguments galore).  Last word hasn't yet been spoken,
but pendulum is swinging towards introns-late (IMHO).

Will Fischer				

Biology Department                      wfischer at indiana.edu
Jordan Hall                             Lab:    812-855-2549
Indiana University                      FAX:    812-855-6705
Bloomington, Indiana 47405 USA          

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