RNAse III

As the discoverers of the enzyme suggest (129), RNase III of E. coli will be an important tool in studies of the function and structure of RNA ... 

Knight, S.W. and Bass, B.L. (2001) A role for the RNase III enzyme DCR-1 in RNA interference and germ line development in Caenorhabditis elegans. Science 293, 2269-2271. 

Yang, D., Buchholz, F., Huang, Z., Goga, A., Chen, C.Y., Brodsky, F.M. and Bishop, J.M. (2002) Short RNA duplexes produced by hydrolysis with Escherichia coli RNase III mediate effective RNA interference in mammalian cel Is. Proceedings of the National Academy of Sciences USA 99, 9942-9947. 

E. coli has seven rRNA transcription units, each containing one copy each of the 23S, 16S and 5S rRNA genes as well as one to four tRNA genes. Transcription produces a 30S pre-rRNA transcript. This folds up to form stem-loop structures, ribosomal proteins bind, and a number of nucleotides become methylated. The modified pre-rRNA transcript is then cleaved at specific sites by RNase III and the ends are trimmed by ribonucleases M5, M6 and M23 to release the mature rRNAs. 

One possibility is that the double-strand RNA-processing activity of RNase III, an endoribonuclease involved in the rate-limiting first cleavage step of mRNA degradation (Chang et al., 1991), is potentially lethal to E. coli, and the... 

RNase III is lethal to the cell at the absence of wild type suhB dnaB 121 forms functional hexamer... 

Fig. 2 The RNAi mechanisms. In the cytoplasm, dsRNA is cleaved into 22nt fragments called small interfering RNAs (siRNAs) by the RNase III t5rpe enzyme Dicer. The siRNAs are unwound and serve either as primers for RdRP (left) in some taxa (e.g., plants and nematodes) or, more universally, as guide strand in the RISC (right). RdRP creates additional dsRNA, amplifying the substrate for Dicer. RISC cleaves the target RNA in the region bound by the siRNA guide.

Fig. 3 The microRNA (miRNA) pathway. An miRNA is first transcribed as part of an imperfect hairpin in a longer Pol II transcript with 5 cap (open circle) and polyadenosine tail (AAAAAA). The hairpin (pre-miRNA) is removed from the transcript by the nuclear RNase III type enzyme Drosha and its partner, DGCR8. Exportin-5 (Xpo-5) transports the resulting by-product to the cytoplasm, where Dicer liberates a short-lived miRNA duplex. The duplex is unwound, and one strand enters RISC. If the miRNA is mismatched to its target (left), it does not induce cleavage, but may inhibit translation. With a perfect or near-perfect match (right), the target RNA is destroyed by RISC.

Basyuk E, Suavet F, DogUo A, Bordonne R, Bertrand E (2003) Human let-7 stem-loop precursors harbor features of RNase III cleavage products. Nucleic Acids Res 31(22) 6593-6597... 

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