Enzymatic conversion of long DNA to small DNA fragments for the construction of short hairpin RNA expression libraries.

Several techniques to enzymatically construct a short hairpin RNA (shRNA) expression library have been reported as tools for comprehensive genetic analyses by RNA interference. Our technique constructs an shRNA expression library from 25- to 35-bp DNA fragments by fragmenting given double-stranded DNA (dsDNA). We compared the following two procedures to efficiently prepare such small DNA fragments: one is the cleavage of dsDNA with deoxyribonuclease I (DNase I) in the presence of Mn(2+) followed by blunting with T4 DNA polymerase, and the other is the introduction of nicks with DNase I in the presence of Mg(2+) followed by blunting with the Klenow fragment. Consequently, the latter yielded the DNA fragments more efficiently. However, these DNA fragments were contaminated with fused DNA fragments that had originated from two regions of original dsDNA. Therefore, we used single-strand-specific exonucleases and succeeded in suppressing the production of such fused DNA fragments. Our technique allows the efficient conversion of given dsDNA to small DNA fragments.

A novel inhibitor stabilizes the inactive conformation of MAPK-interacting kinase 1.

Mitogen-activated protein kinase (MAPK)-interacting kinases 1 (Mnk1) and 2 (Mnk2) modulate translation initiation through the phosphorylation of eukaryotic translation initiation factor 4E, which promotes tumorigenesis. However, Mnk1 and Mnk2 are dispensable in normal cells, suggesting that the inhibition of Mnk1 and Mnk2 could be effective in cancer therapy. To provide a structural basis for Mnk1 inhibition, a novel Mnk1 inhibitor was discovered and the crystal structure of Mnk1 in complex with this inhibitor was determined. The crystal structure revealed that the inhibitor binds to the autoinhibited state of Mnk1, stabilizing the Mnk-specific DFD motif in the DFD-out conformation, thus preventing Mnk1 from switching to the active conformation and thereby inhibiting the kinase activity. These results provide a valuable platform for the structure-guided design of Mnk1 inhibitors.

Genetically engineered active Qbeta replicase in rabbit reticulocyte cell-free system: a fusion protein of EF-Tu and EF-Ts is functional as the subunit of Qbeta replicase.

Qbeta replicase functioning in Escherichia coli is an RNA-dependent RNA polymerase composed of one phage-coded subunit and three host-coded proteins: ribosomal protein S1, and protein elongation factors EF-Tu and EF-Ts. Qbeta replicase lacking ribosomal protein S1 (alpha-less replicase) is capable of replicating some small RNAs. We attempted to create functional alpha-less replicase by co-expression of the mRNAs that code for the subunits of alpha-less replicase in a rabbit reticulocyte cell-free translation system. Replicase activity, however, could not be detected when both EF-Tu and EF-Ts were co-expressed with the phage-coded subunit. On the other hand, active alpha-less replicase was obtained when an EF-Ts-EF-Tu fusion protein was co-expressed with the phage-coded subunit. Consequently, we succeeded in generating genetically engineered active alpha-less Qbeta replicase which functions in a eukaryotic cell-free system.

Uniform amplification of multiple DNAs by emulsion PCR.

When several DNAs are amplified by PCR in one PCR tube, biased amplification is known to occur because amplification efficiency differs from one DNA to another. Therefore, we conducted PCR in the water in oil-emulsion (W/O emulsion) to examine whether the procedure allows the uniform amplification of several DNAs. In the amplification of a model library consisting of two clones, the emulsification of the PCR mixture successfully reduced the difference in its amplification efficiency to approximately one-seventh the value obtained without emulsification. Furthermore, we conducted repeated PCR to amplify a model library consisting of ten short hairpin RNA (shRNA) expression vectors as a model experiment for gene discovery using an shRNA expression library. Consequently, the emulsification of the PCR mixture successfully reduced PCR bias. Our results indicate that emulsion PCR is capable of uniformly amplifying libraries of shRNA, ribozyme, cDNA, and others, and is useful also for gene discovery using these libraries.

A purine at +2 rather than +1 adjacent to the human U6 promoter is required to prepare effective short hairpin RNAs.

The human U6 (hU6) promoter is widely used to express short hairpin RNAs (shRNAs) in mammalian cells. To verify the validity of the generalized concept-the hU6 promoter essentially requires a purine (usually guanine) at +1 for transcription, we enzymatically constructed an arbitrary shRNA library with the following features: (1) to have any one of adenine, cytosine, guanine, and thymine at the site; (2) to comprise shRNAs of 25-30 nucleotides in stem length which are transcribed through the promoter. cDNA of the catalytic subunit of cAMP-dependent protein kinase (PKACalpha) was used as material for library construction. We then used luciferase reporter cell lines to screen shRNAs which effectively reduced PKACalpha activity. Consequently, a purine was mostly present at +2, not at +1, of the clones isolated, suggesting that a purine at +2 rather than +1 adjacent to the hU6 promoter provides effective shRNAs for target gene silencing.

A technique to enzymatically construct libraries which express short hairpin RNA of arbitrary stem length.

Short interfering RNAs (siRNAs) and short hairpin RNAs (shRNAs) usually used for RNA interference (RNAi) are double-stranded RNAs (dsRNAs) of 21 base pairs. However, siRNAs and shRNAs of longer stem length have been reported to show more potent gene silencing. Here, we report a new technique to enzymatically construct shRNA libraries containing clones from firefly luciferase cDNA and Jurkat cDNA. The technique allowed the efficacious generation of shRNAs of arbitrary stem length as desired, providing the clones which potently silenced the specified gene expression and presenting a high efficiency rate of gene silencing. Our results indicate that the technique permits the rapid, efficient, and low-cost preparation of genomewide shRNA expression libraries not only for humans and mice but also for sorts of biological species and that the relevant libraries are applicable for the search of genes related to phenotype changes and of new targets for drug discovery.