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
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