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ISOLATION, CHARACTERIZATION AND EXPRESSION OF TOMATO SUPEROXIDE DISMUTASE GENES – AN UPDATE AND COMPARISON WITH SIMILAR GENES FROM OTHER PLANTS
The latter compound is metabolized further by a cascade of enzymes.
Plants contain two groups of SODs, one group located in mitochondria, contains Mn; the other group contains Cu and Zn proteins and reside either in the chloroplasts or in the cytosol.
The physiology of plant SODs, especially of the chloroplast-located Cu, Zn enzyme, was studied extensively but the molecular biology of these enzymes was handled extensively only during the last two or three years.
The cDNAs of chloropalst-located SODs from several dicots (tomato, petunia, pea) were isolated and sequenced.
All were found to encode a transit peptide having the putative role of leading the enzyme into the chloroplast but the deduced amino-acid sequence varied extensively between the analyzed cDNAs.
On the otherhand there is great homology (90 per cent or more) between the amino-acid sequences of the mature proteins.
The cDNAs of two cytosolic SODs were sequenced up to present: from maize and from tomato.
The deduced amino-acid sequences of these cDNAs showed 80% homology; less homology was revealed between the amino-acid sequence of the cytosolic and the chloroplast-located enzyme of tomato, although all Cu,Zn SODs share conserved sequences.
The availability of these cDNAs permitted studies on the expression of the respective genes.
Such studies in tomato indicated differential expression of the genes coding for either the cytosolic or the chloropalst-located enzymes.
The differences were revealed in specific tomato tissue, during ontogeny and following exposure to specific physical or chemical environments.
The cDNAs of tomato served also, by RFLP techniques, to locate the cytosolic and chloroplast SOD genes to chromosome 1 and 11, respectively.
In addition the cDNA clones were utilized to construct expression vectors for direct and Agrobacterium mediated transformations in order to obtain transgenic plants with additional SOD genes.