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Plant Molecular Biology; Role of Small RNAs in RNA Silencing, Host Defense and Development
Small RNAs have emerged as important regulators of plant defense and development. Small interfering RNAs (siRNAs) are ~21-24 nucleotides in length and mediate RNA silencing. Double-stranded RNA generated during viral replication or derived from aberrant transgenes is digested into siRNAs that direct sequence specific degradation of the corresponding viral or transgene RNA. RNA silencing functions, therefore, to protect the host from viral infection and the expression of aberrant transgenes. SiRNAs or siRNA-like RNAs have also been implicated in transcriptional gene silencing and chromosome elimination.
MicroRNAs (miRNAs) constitute another class of small RNAs. These small RNAs are encoded in the genomes of plants and animals and are processed from highly structured precursors. Although hundreds of these miRNAs have been identified in the last year, the functions of only three have been elucidated. Interestingly, these miRNAs mediate the expression of important developmental regulators. Two miRNA regulate gene expression at the level of translation, and one miRNA appears to alter mRNA stability. Considering the large number of miRNA genes in diverse species, it is likely that some miRNAs regulate gene expression at other levels, such as transcription, mRNA localization, or mRNA processing.
The biogenesis of miRNA and siRNAs share some features but also differ in certain respects. Both are cleaved from highly structured or double-stranded RNA precursors by an RNase III known as Dicer, and members of the agonaunte gene family have been implicated in the formation and/or function of both siRNAs and miRNAs. However, the siRNA and miRNA biogenesis pathways are distinct in that the viral suppressor of silencing, helper component proteinase (Hc-Pro), eliminates siRNAs but enhances miRNA accumulation. Furthermore, there are different genetic requirements for the biogenesis of siRNAs and miRNAs.
Furture experimentation will employ viral suppressors of silencing such as Hc-Pro and mutational analysis to dissect the mechanisms controlling the biogenesis and function of siRNA and miRNAs.
Marathe, R.P., Smith, T.H., Radhamani, A., Bowman, L.H., Fagard, M., Mourrain, H., Vaucheret,H., and Vance, V. (2000). Plant viral suppressors of post-transcriptional silencing do not suppress transcriptional silencing. Plant Journal 22, 51-59.
Anandalakshmi, R., Marathe, R., Ge, X., Herr, J.M., Mallory, A., Mau, C., Pruss, G., Bowman, L.H., and Vance, V.B. (2000) A calmodulin-related protein from tobacco suppresses post-transcriptional gene silencing. Science290, 142-144.
Thames, E.L., Newton, D.A., Black, S.A., and Bowman, L.H. (2000). Role of mRNA stability and translation in the expression cytochrome c oxidase during mouse myoblast differentiation:instability of the mRNA for the liver isoform of subunit VIa. Biochem. J. 351, 133-142.
Mallory, A.C., Ely, L., Smith, T.H., Marathe, R., Anandalakshmi, R.,Fagard, M., Vaucheret, H., Pruss, G., Bowman, L.H. , and Vance, V.B. (2001) HC-Pro suppression of transgene silencing eliminates the small RNAs but not transgene methylation or the mobile signal. Plant Cell13, 571-583.
Dewey, M. J., Ennis, T.M., and Bowman, L.H. (2001) cDNA cloning and expression of the mouse Na/H antiporter (NHE-1) and a potential splice variant. Mol Biol Rep 28, 111-117.
Mallory, A., Parks, G., Endres, M., Baulcombe, D., Bowman, L.H. , Pruss, G.J., and Vance V.B. (2002) The amplicon plus system for high level expression of transgenes. Nat Biotechnol 20, 622-5.
Mallory, A.C., Reinhart B.J., Bartel, D.B., Vance, V.B. and Bowman, L.H. (2002) A viral suppressor of RNA silencing differentially regulates the accumulation of short interfering RNAs and microRNAs in tobacco. Proc Natl Acad Sci USA 99, 15228-33.
Mallory, A.C., Mlotshwa, S., Bowman, L.H. and Vance, V.B. (2002) The Capacity of Transgenic Tobacco to Send a Systemic RNA Silencing Signal Depends on the Nature of the Inducing Transgene. (Submitted to Plant Journal).