Investigations into the use of maize streak virus as a gene vector
Doctoral Thesis
1997
Permanent link to this Item
Authors
Supervisors
Journal Title
Link to Journal
Journal ISSN
Volume Title
Publisher
Publisher
University of Cape Town
Department
Faculty
License
Series
Abstract
This thesis describes investigations into the potential use of the Subgroup I geminivirus, maize streak virus (MSV), as a gene vector. These involved testing MSV-based replicons in transgenic cell lines, in transient expression assays in maize cells and in an infectious gene expression system in plants. MSV vectors which contained three different versions of a bar (bialaphos resistance) expression cassette in place of the viral movement and coat protein genes were used to generate transformed maize cell lines. A high proportion of these contained MSV-based episomes at high copy number. However, embryogenic maize tissue of the Hill line was not regenerable when an MSV-based replicon was present, possibly due to toxicity of the viral replication associated protein. In non-regenerable Black Mexican sweetcorn cell lines some of the MSV-bar episomes, which ranged in size from 3.15 kb to 4.78 kb, replicated for periods of over two years, and appeared structurally stable. The cellular levels of the bar gene product, phosphinothricin acetyl transferase (PAT), were significantly enhanced in lines where the gene was amplified by linkage to an MSV replicon in comparison with lines where the same gene was not amplified. Northern blot analysis also showed that higher levels of bar mRNA were produced in lines where the gene was amplified. However, the 3- to 5-fold enhancement in gene expression was less than was anticipated, based on results from similar Subgroup ill geminivirus-based transgene amplification systems. Several mutants of the MSV genome were generated to investigate the extent to which genome amplification contributes to the expression of the viral coat protein gene. The introduction of an Ncol restriction site at the start of the coat protein gene facilitated fusion of the gus marker gene with the coat protein upstream transcription and translation regulatory sequences. In one viral construct the plus strand origin of replication was inactivated by insertion of a short oligonucleotide; in another, the viral rep gene was inactivated by a frameshift mutation. These constructs were used to show that the MSV coat protein promoter has low, but measurable constitutive activity in the absence of genome amplification, but that viral replication enhances coat protein expression about 45-fold. I found no evidence for Rep-mediated transactivation of the coat protein promoter.
Description
Bibliography: pages 190-214.
Keywords
Reference:
Palmer, K. 1997. Investigations into the use of maize streak virus as a gene vector. University of Cape Town.