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The copper-zinc superoxide dismutase gene from Drosophila melanogaster : attempts to clone the gene using two mixed sequence oligonucleotide probes Seto, Nina Oi Ling


Superoxide dismutase is an enzyme which scavenges superoxide radicals and is thought to be a longevity determinant, as there exists a positive correlation between superoxide dismutase concentration and maximum life span potential. The cytosolic CuZn superoxide dismutase in D. melanogaster has been purified and sequenced, but the gene has not been cloned. However, when it is available the CuZn SOD gene may be reintroduced into the Drosophila genome via the P-element transformation system so its effects on the life span potential of Drosophila may be studied. This study describes attempts to clone the CuZn SOD gene from D. melanogaster using two mixed sequence oligonucleotide probes. The SI probe corresponds to amino acids 43-48 of the protein sequence and contains 128 different oligonucleotide sequences representing all possible codon combinations predicted from the amino acid sequence. The GT3 probe is targeted to amino acids 90-95 of the protein. In this probe, deoxyguanosine was placed in positions where all four nucleotides may occur to decrease probe heterogeneity. The probes were used to screen D. melanogaster Canton-S and Oregon-R genomic lambda libraries. Three positive clones isolated from the Canton-S library had identical nucleotide sequence in the GT3 probe binding region, and sequencing of the probe binding site revealed that one member of the GT3 probe had formed a 15 bp duplex with the phage DNA. Screening of the Oregon-R library produced four clones which hybridized with both GT3 and S1 probes. When these phage DNA were hybridized to polytene chromosomes by in situ hybridization, none mapped to 68AB on the third chromosome, the location of the CuZn SOD gene. These results suggest that modification of the classical strategy used in this study is necessary, and implications on probe design are discussed.

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