Evolutionary Genomics of MicroRNAs and Their Relatives

A. Tanzer, M. Riester, J. Hertel, C. I. Bermudez-Santana, J. Gorodkin, I. L. Hofacker and P. F. Stadler
In Gustavo Caetano-Anolles, editor, Evolutionary Genomics. Wiley, in press.


MicroRNAs (miRNAs) are an abundant class of small noncoding RNA (ncRNA) genes which were found in eukaryotes, in particular in metazoans and plants, and in their viruses. MicroRNA research has come a long way, since the first discoveries of lin-4 [102] and let-7 [147] in Caenorhabditis elegans. The turn of the century brought the realization that miRNAs form a large new class of ncRNAs [101, 97, 95] that provide a ubiquitous and powerful mechanism for RNA-mediated control of gene expression. The miRBase [63], a comprehensive database collecting published miRNAs as well as assigning unique names [6] to novel ones, started with only 218 sequences (v1.0, December 2002) and now lists 6396 entries in the current version 11.0 (April 2008). Today (08/05/2008) there are 3233 publications about miRNAs in PubMed of which 755 are reviews. These numbers might ilustrate the impact of this field of research on our understanding of the infomation encoded by the fast majority of genomic sequences and transcribed units. MicroRNAs were the first small regulatory RNAs found in animals, but turned out not to be the only ones. During the past few years, a variety of additional classes were detected, many of which share functional properties and processing machinery. In the following section we will address those similarities as well as differences by outlining biogenesis and function.