An extensive class of small RNAs in Caenorhabditis elegans

The lin-4 and let-7 antisense RNAs are temporal regulators that control the timing of developmental events in Caenorhabditis elegans by inhibiting tra nslation of target mRNAs. let-7 RNA is conserved among bilaterian animals, suggesting that this class of small RNAs [microRNAs (miRNAs)] is evolutiona rily ancient. Using bioinformatics and cDNA cloning, we found 15 new miRNA genes in C. elegans. Several of these genes express small transcripts that vary In abundance during C. elegans larval development, and three of them h ave apparent homologs in mammals and/or insects. Small noncoding RNAs of th e miRNA class appear to be numerous and diverse.