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L K Johansen

Publications and source records attributed to L K Johansen.

8 recordsLinked to original sources
PubMed2001

Silencing on the spot. Induction and suppression of RNA silencing in the Agrobacterium-mediated transient expression system.

The Agrobacterium-mediated transient expression assay in intact tissues has emerged as a rapid and useful method to analyze genes and gene products in plants. In many cases, high levels of active protein can be produced without the need to produce transgenic plants. In this study, a series of tools were developed to enable strong or weak induction of RNA silencing and to suppress RNA silencing in the absence of stable transgenes. Transient delivery of a gene directing production of a double-stranded green fluorescent protein (GFP) transcript rapidly induced RNA silencing of a codelivered GFP reporter gene, effectively preventing accumulation of GFP protein and mRNA. RNA silencing triggered by the strong dsGFP inducer was partially inhibited by the tobacco etch virus silencing suppressor, P1/HC-Pro. In the absence of the strong double-stranded GFP inducer, the functional GFP gene served as a weak RNA silencing inducer in the transient assay, severely limiting accumulation of the GFP mRNA over time. The weak silencing induced by the GFP gene was suppressed by P1/HC-Pro. These results indicate RNA silencing can be triggered by a variety of inducers and analyzed entirely using transient gene delivery systems. They also indicate that RNA silencing may be a significant limitation to expression of genes in the Agrobacterium-mediated transient assay but that this limitation can be overcome by using RNA silencing suppressors.

Cloning, Molecular↗
PubMed2000

The RNA encompassing the internal ribosome entry site in the poliovirus 5' nontranslated region enhances the encapsidation of genomic RNA.

Poliovirus replicons were constructed which contain the internal ribosome entry site (IRES) of encephalomyocarditis virus (EMCV) substituted for the poliovirus IRES. To monitor gene expression and encapsidation, the gene encoding firefly luciferase was substituted for the P1 gene. Replicons can be encapsidated following serial passage in the presence of a recombinant vaccinia virus, VV-P1, which expresses the poliovirus P1 protein following infection. Encapsidation of the wild-type replicon (PV-Luc) was accomplished at either 33 or 37 degrees C; the lower temperature actually resulted in greater amounts of encapsidated replicon. In contrast, the replicon with the EMCV IRES element (EMCV-Luc) was not efficiently encapsidated at 37 degrees C and, following serial passage with VV-P1 at 37 degrees C, was not amplified. EMCV-Luc was efficiently encapsidated, however, following serial passage with VV-P1 at 33 degrees C. Using the encapsidated EMCV-Luc obtained at 33 degrees C, we found that cells infected with EMCV-Luc at 33 or 37 degrees C produced similar amounts of luciferase. Encapsidated EMCV-Luc and PV-Luc had similar thermal stability at 33 and 37 degrees C. A single-round encapsidation analysis revealed that less EMCV-Luc was encapsidated at 37 than at 33 degrees C; less EMCV-Luc was encapsidated at 33 degrees C compared to PV-Luc at either 37 or 33 degrees C. The results of our studies suggest that in addition to influencing translation/replication, the IRES region of poliovirus can function to enhance encapsidation.

5' Untranslated Regions↗
PubMed2000

Inherent instability of poliovirus genomes containing two internal ribosome entry site (IRES) elements supports a role for the IRES in encapsidation.

Previous studies have described poliovirus genomes in which the internal ribosome entry (IRES) for encephalomyocarditis virus (EMCV) is positioned between the P1 and P2-P3 open reading frames of the poliovirus genome. Although these dicistronic poliovirus genomes were replication competent, most exhibited evidence of genetic instability, and the EMCV IRES was deleted upon serial passage. One possible reason for instability of the genome is that the dicistronic genome was at least 108% larger than the wild-type poliovirus genome, which could reduce the efficiency of encapsidation. To address this possibility, we have constructed dicistronic poliovirus replicons by substituting the EMCV IRES and the gene encoding luciferase in place of the poliovirus P1 region; the resulting dicistronic replicons are smaller than the wild-type poliovirus genome. One dicistronic genome was constructed in which the poliovirus 5' nontranslated region was fused to the gene encoding luciferase, followed by the complete EMCV IRES fused to the P2-P3 region of the poliovirus genome (PV-Luc-EMCV). A second dicistronic genome, EMCV-Luc-PV, was constructed with the first 108 nucleotides of the poliovirus genome fused to the EMCV IRES, followed by the gene encoding luciferase and the poliovirus IRES fused to the remaining P2-P3 region of the poliovirus genome. Both dicistronic replicons expressed abundant luciferase following transfection of in vitro-transcribed RNA into HeLa cells at 30, 33, or 37 degrees C. The luciferase activity detected from PV-Luc-EMCV increased rapidly during the first 4 h following transfection and then plateaued, peaking after approximately 24 h. In contrast, the luciferase activity detected from EMCV-Luc-PV increased for approximately 12 h following transfection; by 24 h posttransfection, the overall levels of luciferase activity were similar to that of PV-Luc-EMCV. To analyze encapsidation of the dicistronic replicons, we used a system in which the capsid protein (P1) is provided in trans from a recombinant vaccinia virus (VV-P1). The PV-Luc-EMCV replicon was unstable upon serial passage in the presence of VV-P1, with deletions of the EMCV IRES region detected even during the initial transfection at 37 degrees C. Following serial passage in the presence of VV-P1 at 33 or 30 degrees C, we detected deleted genomes in which the luciferase gene was fused with the P2-P3 genes of the poliovirus genome so as to maintain the translational reading frame. In contrast, the EMCV-Luc-PV replicon was genetically stable during passage with VV-P1 at 33 or 30 degrees C. The encapsidation of EMCV-Luc-PV was compared to that of monocistronic replicons encoding luciferase with either a poliovirus or EMCV IRES. Analysis of the encapsidated replicons after four serial passages with VV-P1 revealed that the dicistronic replicon was encapsidated more efficiently than the monocistronic replicon with the EMCV IRES but less efficiently than the monicistronic replicon with the poliovirus IRES. The results of this study suggest a genetic predisposition for picornavirus genomes to contain a single IRES region and are discussed with respect to a role of the IRES in encapsidation.

5' Untranslated Regions↗
PubMed1996

Phage-display libraries of murine and human antibody Fab fragments.

We provide efficient and detailed procedures for construction, expression, and screening of comprehensive libraries of murine or human antibody Fab fragments displayed on the surface of filamentous phage. In addition, protocols for producing and using ultra-electrocompetent cells, for producing Fab phages from libraries, and for selecting antigen binders by panning are presented. The latter protocol includes a procedure for trypsin elution of bound phage.

Animals↗
PubMed1993

Efficient method for constructing comprehensive murine Fab antibody libraries displayed on phage.

We have developed efficient methodologies for construction and expression of comprehensive phage display libraries of murine Fab antibody fragments in E. coli cells. Our methods optimize several critical steps of the polymerase chain reaction (PCR) amplification of transcripts of the re-arranged immunoglobulin genes and of their subsequent assembly and expression: Firstly, we have designed exhaustive sets of PCR primers of low degeneracy for the amplification of transcripts of the Fab region of the heavy and light-chain genes. These primers proved effective in amplification of Fab gene fragments from a large panel of hybridoma cell lines of different specificity and family sub-type. Secondly, we have developed a 'jumping PCR' technique that effectively assembled and recombined the amplified heavy and light-chain gene fragments into a bi-cistronic operon. Thirdly, we have constructed expression vectors for insertion of the combinatorial Fab gene-cassette in fusion with a truncated version of the phage surface protein, gIIIp. The heavy chain and the light chain-gIII fusion are transcribed as a polycistronic mRNA from the lacZ promoter and efficient transcriptional control is provided by wildtype lacI present on the vector. The utility of the system was demonstrated by isolating several antigen-binding clones from hybridomas and libraries made from immunized mice.

Animals↗