chloroplast transformation, gene regulation, RNA binding proteins, type I diabetes, amino acid metabolism
Genetic modification of plastids in the model plant tobacco (Nicotiana tabacum) has demonstrated that numerous foreign gene products can accumulate to high levels in this setting. Plastid biotechnology is maturing to encompass the improvement of food and feed species and the production of biopharmaceutical proteins for oral delivery necessitating development of stable transplastomic edible plants. In the interest of establishing an edible platform we have investigated the use of native and foreign regulatory elements in relation to foreign gene expression in plastids. Multiple sequence alignments of intergenic regions for 20 species of angiosperm showed that despite 95% identity in the coding region, identity in the psbA upstream region is 59% across all taxa examined, other gene coding regions displayed sequence identity of 80-97%, whereas the non-coding regions were 45-79% suggesting that our physical data can be extrapolated beyond the model presented. We found that by exchanging psbA untranslated regions (UTRs) between N. tabacum and lettuce (Lactuca sativa), the expression of the CTB-proinsulin (CTB-Pins) monocistronic transcript declined by 84% and foreign protein accumulation was reduced by as much as 97% in mature leaves. Polyribosome association assays suggest that ribosome-free transgenic transcripts are stabilized where the native UTR is employed. RNA EMSA revealed that binding proteins interacted with psbA 5' UTRs in a species specific manner and the half life of the L. sativa 5'UTR-CTB-Pins mRNA was reduced by 3.7 fold in N. tabacum stromal extracts. Our data indicate that the use of species-specific regulatory elements could lead to establishment of reproducible plastid transformation in desirable target species such as L. sativa. Using transplastomic L. sativa for oral delivery of bioencapsulated CTB-Pins we delayed the onset of diabetes in NOD mice when retinyl acetate supplement was provided compared to untouched mice. In this 30 week study we monitored blood glucose levels and evaluated the in vitro suppressive capacity of regulatory T cells isolated from diabetic mice. Whether delay or prevention was achieved appeared to be a function of antigen dose as high dose resulted in a nine week delay of onset while low dose reduced the incidence of diabetes by 36%. In addition we have evaluated metabolic engineering in the N. tabacum model where we generated cis-genic lines expressing nucleus-encoded methionine pathway enzymes in plastids. Transplastomic expression of Cystathionine gamma-Synthase led to a three-fold increase in enzyme activity and a doubling of methionine content in leaves without a deleterious phenotype. In exploring molecular mechanisms supporting gene expression in plastids and applying transplastomic technology to real human problems this work seeks address the potential of plastid biotechnology for improvement of commodity crops and production of biopharmaceuticals.
Doctor of Philosophy (Ph.D.)
Burnett College of Biomedical Sciences
Length of Campus-only Access
Doctoral Dissertation (Open Access)
Ruhlman, Tracey, "Determinants Of Chloroplast Gene Expression And Applications Of Chloroplast Transformation In Lactuca Sativa And Nicotiana Tabacum" (2009). Electronic Theses and Dissertations. 3825.