MicroRNAs: At the Root of Plant Development?

Abstract

Although most genes use RNA in the form of mRNA as a coding intermediate for protein production, there are many genes whose final products are RNA. These noncoding RNAs range from the familiar transfer and ribosomal RNAs to the more recently discovered regulatory RNAs. One type of regulatory RNA was first discovered during the study of nematode larval development. Two approximately 22-nucleotide (nt) RNAs (the lin-4 and let-7 RNAs) control developmental timing by binding to their respective mRNA targets and preventing productive use of these messages, perhaps by attenuating translation (Lee et al., 1993; Pasquinelli and Ruvkun, 2002). The let-7 RNA was found broadly throughout bilateral animals, including humans, suggesting that these two riboregulators were more than oddities of worm larval development (Pasquinelli et al., 2000). In 2001, it was discovered that these two RNAs are members of a large class of 21- to 24-nt noncoding RNAs, called microRNAs (miRNAs), found in nematodes, fruitflies (Drosophila melanogaster), and humans (Lagos-Quintana et al., 2001; Lau et al., 2001; Lee and Ambros, 2001). Recent computational and molecular analyses indicate that humans have over 200 miRNA genes, nearly all of which are conserved in mice, and at least 80% of which are conserved in fish (Lim et al., 2003a). Although the functions of nearly all metazoan miRNAs are unknown, their abundance and evolutionary conservation, together with the analogy to the two founding nematode miRNAs, which are posttranscriptional gene regulators, suggests that an important mode of metazoan gene regulation had gone virtually undetected until just recently.