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Nucleotide sequence and expression of human chromosome 21-encoded superoxide dismutase mRNA.

Cytoplasmic superoxide dismutase (SOD-1; EC 1.15.1.1) is encoded by human chromosome 21. The SOD-1 gene locus is located at chromosomal region 21q22, which is involved in Down syndrome. cDNA clones containing sequences of human SOD-1 were previously isolated. In the present study the nucleotide sequence of one clone, designated pS61-10, was determined. It contains 459 nucleotides representing the entire coding region and 95 nucleotides of the 3' untranslated region. In human cells two poly(A)-containing SOD-1 RNAs of 0.7 and 0.5 kilobases were detected. These two species are also present in monkey cells, whereas mouse cells contain only a 0.5-kilobase RNA. In a mouse/human hybrid line that contains chromosome 21 as the only human chromosome, the two human SOD-1 RNAs were detected, indicating that both are encoded by this chromosome. These RNAs were found in poly(A)-containing polysomal RNA and were translated in vitro to SOD-1 polypeptide; they are therefore functional mRNAs. In normal human fibroblasts 0.002-0.006% of the poly(A)-containing RNA was SOD-1 RNA. The level in monosomic 21 cells was 70% of this value and the level in fibroblasts from Down syndrome patients was about 2 times higher than normal.