(1)Department of Environmental Information
and Department of Molecular Biology
Keio University, 5322 Endo, Fujisawa, 252, JAPAN
(2)Department of Molecular Biology, Keio University
(3)Department of Biochemistry, Stanford University
Computer analyses of the entire Genbank database were conducted
to examine correlation between splicing sites and codon positions in
reading frames. Intron insertion patterns (i.e., splicing site locations
with respect to codon positions) have been analyzed for all of the 64
codons of all the eukaryote taxonomical groups: primates, rodents,
mammals, vertebrates, invertebrates and plants.
We found that reading frames are interrupted by an intron at a
codon boundary (as oppose to the middle of a codon) significantly more
often than expected. This observation is consistent with the {\it
exon shuffling hypothesis}, because exons that end at codon boundaries
can be concatenated without causing a frame shift and thus are
evolutionarily advantageous.
On the other hand, when introns interrupt at the middle of
codons, they exist in between the first and second bases much more
frequently than the second and third bases, despite the fact that
boundaries between the first and second bases of codons are generally
far more important than those between the second and third bases. The
reason is not clear and yet to be explained.
We also show that the length of an exon is a multiple of 3 more
frequently than expected. Furthermore, the total length of two
consecutive exons is also more frequently a multiple of 3.
All the observations above are consistent with the recently published
results by Long, Rosenberg and Gilbert (1995).