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MBE Advance Access published online on September 15, 2006
Molecular Biology and Evolution, doi:10.1093/molbev/msl111
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© The Author 2006. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution. All rights reserved. For permissions, please e-mail: journals.permissions@oxfordjournals.org
Accepted September 7, 2006

Research Article

Intron Size, Abundance and Distribution Within Untranslated Regions of Genes

Xin Hong 1, Douglas G. Scofield 1 *, and Michael Lynch 1

1 Department of Biology, Indiana University, Bloomington, IN 47405

* To whom correspondence should be addressed.
Douglas G. Scofield, E-mail: dgscofie{at}indiana.edu


  Abstract

Most research concerning the evolution of introns has largely considered introns within coding sequences (CDSs), without regard for introns located within untranslated regions (UTRs) of genes. Here, we directly determined intron size, abundance and distribution in untranslated regions of genes using full-length cDNA libraries and complete genome sequences for four species, Arabidopsis thaliana, Drosophila melanogaster, human, and mouse. Overall intron occupancy (introns / exon kbp) is lower in 5' UTRs than CDSs, but intron density (intron occupancy in regions containing introns) tends to be higher in 5' UTRs than in CDSs. Introns in 5' UTRs are roughly twice as large as introns in CDSs, and there is a sharp drop in intron size at the 5' UTR-CDS boundary. We propose a mechanistic explanation for the existence of selection for larger intron size in 5' UTRs, and outline several implications of this hypothesis. We found introns to be randomly distributed within 5' UTRs, so long as a minimum required exon size was assumed. Introns in 3' UTRs were much less abundant than in 5' UTRs. Though this was expected for human and mouse, which have intron-dependent nonsense-mediated decay (NMD) pathways that discourage the presence of introns within the 3' UTR, it was also true for A. thaliana and D. melanogaster, which may lack intron-dependent NMD. Our findings have several implications for theories of intron evolution and genome evolution in general.

Keywords: untranslated region; intron; genome evolution.
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