Expression of a reporter gene is reduced by a ribozyme in transgenic plants

Ribozymes: recent advances in the development of RNA tools

The discovery 20 years ago that some RNA molecules, called ribozymes, are able to catalyze chemical reactions was a breakthrough in biology. Over the last two decades numerous natural RNA motifs endowed with catalytic activity have been described. They all fit within a few well-defined types that respond to a specific RNA structure. The prototype catalytic domain of each one has been engineered to generate trans-acting ribozymes that catalyze the site-specific cleavage of other RNA molecules. On the 20th anniversary of ribozyme discovery we briefly summarize the main features of the different natural catalytic RNAs. We also describe progress towards developing strategies to ensure an efficient ribozyme-based technology, dedicating special attention to the ones aimed to achieve a new generation of therapeutic agents.

Episomal expression of a hammerhead ribozyme directed against plum pox virus

Two related antisense RNAs directed against plum pox virus (PPV) were expressed episomally in Nicotiana clevelandii by infection with recombinant potato virus X (PVX). One recombinant PVX expressed an ordinary PPV antisense RNA of about 400 nucleotides, while the other expressed a related antisense RNA that carried the catalytic domain of a hammerhead ribozyme. Inoculation with the latter recombinant PVX resulted in the accumulation of ribozyme RNA that was catalytically active when tested in vitro with a PPV substrate RNA. Plants that had been inoculated with recombinant PVX viruses, expressing either PPV-directed antisense or ribozyme sequences or GUS RNA as a control, were challenged with PPV by a sequential second inoculation. In plants that expressed PPV antisense sequences, the appearance of PPV disease symptoms was delayed for 3-5 days. Quantification of PPV 1 week after inoculation showed that the protective effect by the episomally expressed catalytic antisense RNA was stronger than that of the ordinary antisense RNA. However, eventually all plants tested accumulated comparable titers of PPV.

Regulation of CAT protein by ribozyme and antisense mRNA in transgenic mice

Transgenic mouse lines were engineered to express stably antisense mRNA or antisense mRNA containing catalytic ribozyme (rbz) structures complementary to bacterial chloramphenicol acetyltransferase (CAT) gene transcripts. One transgenic line expressed antisense mRNA that specifically targeted full-length CAT coding sequences (ACAT). Another transgenic line expressed full-length antisense CAT mRNA which was modified by mutagensis to include four rbz cassettes (rbz-ACAT) in order to compare antisense versus antisense-rbz function in vivo. Preliminary data were also collected from a transgenic mouse line expressing antisense mRNA targeting 72% of the 5' region of CAT coding sequences (5' ACAT). All constructs contained similar control elements in their design. Promoter elements were derived from the bovine alpha s1-casein gene, while the small t intron and 3' control sequences were derived from SV40. The ability of these various constructs to down-regulate CAT protein levels was compared by analysis of CAT protein production in lactating double-hemizygous transgenic female mice. Every double-hemizygous mouse analysed expressed mRNA from the alpha s1-casein-CAT construct (Clarke et al., 1994) and equivalent levels of mRNA from one of the three antisense constructs. Transgenic mouse lines expressing both ACAT and CAT mRNA down-regulated CAT protein levels by 90% of that found in the CAT only transgenic population. Similarly, double-hemizygous transgenic lines expressing both rbz-ACAT and CAT mRNA regulated CAT protein levels by 87%. Preliminary data suggests that expression of mRNA from 5' ACAT/CAT double-hemizygote mice allowed approximately 67% down-regulation of normal CAT protein levels. We conclude that incorporation of multiple ribozymes within the full-length antisense CAT construct does not enhance the effectiveness of antisense mRNA in the down-regulation of CAT protein production in our system.

Strategies for the suppression of peroxidase gene expression in tobacco. II. In vivo suppression of peroxidase activity in transgenic tobacco using ribozyme and antisense constructs

Several strategies involving the use of antisense and ribozyme constructs in different expression vectors were investigated as methods of suppressing gene expression in planta. We had previously identified an efficiently cleaving ribozyme (Rz), with two catalytic units and 60 nucleotide (nt) of complementary sequence, to the lignin-forming peroxidase of tobacco (TPX). This Rz was cloned behind the 35S CaMV (35S) and nopaline synthase (NOS) promoters, and into a vector utilising the tobacco tyrosine tRNA for expression. For comparison with more traditional antisense strategies, full-length TPX antisense (AS) constructs were also constructed behind the NOS and 35S promoters. Populations of transgenic tobacco containing these constructs were produced and compared to control plants transformed with the vector only. Significant suppression of peroxidase expression in the range of 40-80% was seen in the T0 and T1 populations carrying 35S-AS, 35S-Rz and tRNA-Rz constructs. Co-segregation of the suppressed peroxidase phenotype and the tRNA-Rz transgenes was demonstrated. Northern blot analysis indicated that levels of TPX mRNA were lower in the Rz plants. No evidence of mRNA cleavage was observed and thus it was unclear if the Rz constructs were acting as Rzs in vivo. Transgenic plants containing the tRNA-Rz construct had significantly lower levels of peroxidase than the other transgenic plants. There was no significant difference in levels of suppression of TPX between the short Rz in the 35S vector and the full-length AS constructs. Although peroxidase levels were significantly reduced in transgenic plants carrying 35S-AS, 35S-Rz and tRNA-Rz constructs, no significant difference in lignin levels was observed.

A ribozyme gene and an antisense gene are equally effective in conferring resistance to tobacco mosaic virus on transgenic tobacco

Ribozymes of the hammerhead class can be designed to cleave a target RNA in a sequence-specific manner and can potentially be used to specifically modulate gene activity. We have targeted the tobacco mosaic virus (TMV) genome with a ribozyme containing three catalytic hammerhead domains embedded within a 1 kb antisense RNA. The ribozyme was able to cleave TMV RNA at all three target sites in vitro at 25 degrees C. Transgenic tobacco plants were generated which expressed the ribozyme or the corresponding antisense constructs directed at the TMV genome. Six of 38 independent transgenic plant lines expressing the ribozyme and 6 of 39 plant lines expressing the antisense gene showed some level of protection against TMV infection. Homozygous progeny of some lines were highly resistant to TMV; at least 50% of the plants remained asymptomatic even when challenged with high levels of TMV. These plants also displayed resistance to infection with TMV RNA or the related tomato mosaic virus (ToMV). In contrast, hemizygous plants of the same lines displayed only very weak resistance when inoculated with low amounts of TMV and no resistance against high inoculation levels. Resistance in homozygous plants was not overcome by a TMV strain which was altered at the three target sites to abolish ribozyme-mediated cleavage, suggesting that the ribozyme conferred resistance primarily by an antisense mechanism.

A stable hammerhead structure is not required for endonucleolytic activity of a ribozyme in vivo

Cleavage of a specific target, the mRNA encoding the bacterial neomycin phosphotransferase, by mutant satellite RNA of subterranean clover mottle virus (sSCMoV) ribozymes (Rz) was used to study the role of the hammerhead (Hh) structure in Rz activity in cis and in trans. The bimolecular Rz-target RNA interaction was predicted by computer secondary structure analysis. In vivo, endonucleolytic cleavage was determined in plant protoplasts and compared with in vitro results. Two point mutations within the Hh were studied in detail. A Rz mutant with a point mutation in the most distal nucleotide of the catalytic domain (A14G) showed no endonucleolytic activity in vivo. A second point mutation inside helix II (G11.3C) which destabilizes the helix and, according to thermodynamic calculations, should disrupt the conserved Hh structure, unexpectedly displayed Rz activity in trans in vivo. In vitro, this mutant exhibited an activity similar to the wild-type Rz in cis, but no significant activity in trans. It therefore appears that helix II within the Rz Hh structure is not required in vivo for endonucleolytic activity, nor for stability of the Rz transcript, and that in vitro results are inadequate to predict Rz activity in living cells.