Micro RNAs in animal development.

Micro RNAs (miRNAs) are approximately 22 nucleotide single-stranded noncoding RNA molecules that bind to target messenger RNAs (mRNAs) and silence their expression. This Essay explores the importance of miRNAs in animal development and their possible roles in disease and evolution

Knocking out the Argonautes.

Argonaute proteins are key players in gene silencing involving small RNAs. In this issue, Yigit et al. (2006) report a comprehensive study of Argonautes in the worm that places many of the 27 family members into a complex gene-silencing network

The diverse functions of microRNAs in animal development and disease.

MicroRNAs (miRNAs) control gene expression by translational inhibition and destabilization of mRNAs. While hundreds of miRNAs have been found, only a few have been studied in detail. miRNAs have been implicated in tissue morphogenesis, cellular processes like apoptosis, and major signaling pathways. Emerging evidence suggests a direct link between miRNAs and disease, and miRNA expression signatures are associated with various types of cancer. In addition, the gain and loss of miRNA target sites appears to be causal to some genetic disorders. Here, we discuss the current literature on the role of miRNAs in animal development and disease

Characterization of the Caenorhabditis elegans Tc1 transposase in vivo and in vitro

We have investigated the function of the Tc1A gene of the mobile element Tc1 of Caenorhabditis elegans. Tc1 is a member of a family of transposons found in several animal phyla, such as nematodes, insects, and vertebrates. Two lines of evidence show that Tc1A encodes the transposase of Tc1. First, forced expression of the Tc1A protein in transgenic nematodes results in an enhanced level of transposition of endogenous Tc1 elements. Second, DNase I footprinting and gel retardation assays show that Tc1A binds specifically to the inverted repeats at the ends of the element and that the Tc1A recognition site is located between base pairs 5 and 26 from the ends of Tc1. Functional dissection of the transposase shows the presence of two distinct DNA-binding domains. A site-specific DNA-binding domain is contained within the amino-terminal 63 residues of Tc1A; this region shows sequence similarity to the prokaryotic IS30 transposase. A second, general DNA-binding domain is located between amino acids 71 and 207. Our results suggest that Tc1 is more similar to prokaryotic insertion elements than to eukaryotic transposons such as P elements in Drosophila or Ac and En-1 in plants

Secondary siRNAs result from unprimed RNA synthesis and form a distinct class.

In Caenorhabditis elegans, an effective RNA interference (RNAi) response requires the production of secondary short interfering RNAs (siRNAs) by RNA-directed RNA polymerases (RdRPs). We cloned secondary siRNAs from transgenic C. elegans lines expressing a single 22-nucleotide primary siRNA. Several secondary siRNAs start a few nucleotides downstream of the primary siRNA, indicating that non-RISC (RNA-induced silencing complex)-cleaved mRNAs are substrates for secondary siRNA production. In lines expressing primary siRNAs with single-nucleotide mismatches, secondary siRNAs do not carry the mismatch but contain the nucleotide complementary to the mRNA. We infer that RdRPs perform unprimed RNA synthesis. Secondary siRNAs are only of antisense polarity, carry 5' di- or triphosphates, and are only in the minority associated with RDE-1, the RNAi-specific Argonaute protein. Therefore, secondary siRNAs represent a distinct class of small RNAs. Their biogenesis depends on RdRPs, and we propose that each secondary siRNA is an individual RdRP product

Transposase is the only nematode protein required for in vitro transposition of Tc1

The Tc1 element of Caenorhabditis elegans is a member of the most widespread class of DNA transposons known in nature. Here, we describe efficient and precise transposition of Tc1 in a cell-free system. Tc1 appears to jump by a cut-and-paste mechanism of transposition. The terminal 26 bp of the Tc1 terminal repeats together with the flanking TA sequence are sufficient for transposition. The target site choice in vitro is similar to that in vivo. Transposition is achieved with an extract prepared from nuclei of transgenic nematodes that overexpress Tc1 transposase but also by recombinant transposase purified from Escherichia coli. The simple reaction requirements explain why horizontal spread of Tc1/mariner transposons can occur. They also suggest that Tcl may be a good vector for transgenesis of diverse animal species