No abstract

MicroRNAs (miRNAs) in eukaryotes guide post-transcriptional regulation by means of targeted RNA degradation and translational arrest. They are released by a Dicer nuclease as a 21-24-nucleotide RNA duplex from a precursor in which an imperfectly matched inverted repeat forms a partly double-stranded region. One of the two strands is then recruited by an Argonaute nuclease that is the effector protein of the silencing mechanism. Short interfering RNAs (siRNAs), which are similar to miRNAs, are also produced by Dicer but the precursors are perfectly double-stranded RNA. These siRNAs guide post-transcriptional regulation, as with miRNAs, and epigenetic genome modification. Diverse eukaryotes including fungi, plants, protozoans and metazoans produce siRNAs but, until now, miRNAs have not been described in unicellular organisms and it has been suggested that they evolved together with multicellularity in separate plant and animal lineages. Here we show that the unicellular alga Chlamydomonas reinhardtii contains miRNAs, putative evolutionary precursors of miRNAs and species of siRNAs resembling those in higher plants. The common features of miRNAs and siRNAs in an alga and in higher plants indicate that complex RNA-silencing systems evolved before multicellularity and were a feature of primitive eukaryotic cells.

SummaryMicroRNAs (miRNAs) are small RNAs, 21 to 22 nucleotides long, with important regulatory roles. They are processed from longer RNA molecules with imperfectly matched foldback regions and they function in modulating the stability and translation of mRNA. Recently, we and others have demonstrated that the unicellular alga Chlamydomonas reinhardtii, like diverse multicellular organisms, contains miRNAs. These RNAs resemble the miRNAs of land plants in that they direct site-specific cleavage of target mRNA with miRNA-complementary motifs and, presumably, act as regulatory molecules in growth and development. Utilizing these findings we have developed a novel artificial miRNA system based on ligation of DNA oligonucleotides that can be used for specific high-throughput gene silencing in green algae.

The core protein of the hepatitis B virus, HBcAg, assembles into highly immunogenic virus-like particles (HBc VLPs) when expressed in a variety of heterologous systems. Specifically, the major insertion region (MIR) on the HBcAg protein allows the insertion of foreign sequences, which are then exposed on the tips of surface spike structures on the outside of the assembled particle. Here, we present a novel strategy which aids the display of whole proteins on the surface of HBc particles. This strategy, named tandem core, is based on the production of the HBcAg dimer as a single polypeptide chain by tandem fusion of two HBcAg open reading frames. This allows the insertion of large heterologous sequences in only one of the two MIRs in each spike, without compromising VLP formation. We present the use of tandem core technology in both plant and bacterial expression systems. The results show that tandem core particles can be produced with unmodified MIRs, or with one MIR in each tandem dimer modified to contain the entire sequence of GFP or of a camelid nanobody. Both inserted proteins are correctly folded and the nanobody fused to the surface of the tandem core particle (which we name tandibody) retains the ability to bind to its cognate antigen. This technology paves the way for the display of natively folded proteins on the surface of HBc particles either through direct fusion or through non-covalent attachment via a nanobody.