New class of small RNAs found

Members of a newly identified class of small RNAs may have a biological function that belies their diminutive moniker. Dubbed Piwi-interacting RNAs (piRNAs), the small RNAs bind to members of the Piwi family of proteins, report two independent research groups led by Thomas Tuschl of Rockefeller University and Gregory J. Hannon of Cold Spring Harbor Laboratory (Nature, published online June 4, dx.doi.org/10.1038/nature 04916; dx.doi.org/10.1038/ nature04917). Piwi proteins are related to Argonaute proteins, which are involved in the process of RNA-mediated modulation of gene expression known as RNA interference. The piRNAs are about 30 nucleotides long and were found in mouse testes. The piRNAs are difficult to organize into sequence families, but the DNA sequences that code for different piRNAs have a tendency to cluster at specific spots in the genome, suggesting that the location in the genome is more important than the piRNA sequence. Similar piRNA-encoding clusters were also found in human and rat chromosomes. The function of piRNAs is unknown, but they appear to be involved in the formation of sperm cells.