Phospholipase A2 Introns

Sun Nov 27 20:12:25 EST 1994

My friend pointed out this situation that seems to have interesting
implications, but I have been unable to make heads or tails of it.  I thought I
would ask you to see if anyone had any ideas.
	A lab here at the University of Wyoming sequenced a phospholipase A2
enzyme from the Mojave rattlesnake (John et al., Gene 139:229-234). 
Phospholipase A enzymes are found in species from humans to snakes to bees. 
They serve as digestive enzymes in humans and as neurotoxins in the venoms of
some snakes.
	The mojave toxin is a heterodimeric molecule made up of one basic and
one acidic subunit.  These subunits show high nucleotide identity supporting
the idea that they evolved from a common protein.  However, surprisingly, the
introns of these subunits show higher sequence homology than the exons.  The
introns (plus the 5' and 3' UTR) show 97%, 93%, 85%, 97%, and 97% homology by
region, while the exons show 100%, 71%, 69%, and 66%.  These results seem quite
counterintuitive.  However this closely correlates with Nakashima et al. (PNAS
90:5964-5968) who sequenced 6 phosphlipase A2 isoenzyme genes from the closely
related Habu snake.
	I suspect this high sequence conservation among the introns indicates a
functional importance.  Nakashima et al., however, suggest no function due to
the lack of significant secondary structure found by GENAS system analysis. 
They propose an "accelerated evolution" among the exons.  I am skeptical of
this since I know of no mechanism that accelerates gene mutation rates
(especially between exons vs. introns).
	I would greatly appreciate any feedback on this topic.  Please respond
over the newsgroup or privately.  I am at a loss to explain these results and
can find no similar cases in the literature.

					Kirk A. Adams
                                        Ferdia at uwyo.edu					
					University of Wyoming

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