Repetitive DNA (TGGA)_n 5′ to the human myelin basic protein gene: A new form of oligonucleotide repetitive sequence showing length polymorphism

DNA 5′ to the human myelin basic protein (MBP) gene, mapped to 18q22 → qter, is known to manifest multiallelic DNA length variation with heterozygosity of at least 45%. Isolation of genomic DNA containing the MBP gene first exon and its 5′ flanking region reveals that this polymorphism arises from a 994-bp region of the diverged tandem repeat (TGGA)₂₄₉. This sequence is located from 1082 to 2075 bp upstream of the MBP initiator methionine. The repetitive sequence is 18% diverged from (TGGA)₂₄₉ and from analysis of higher order subsequence reiterations appears to have undergone extensive recombination. The pattern of higher order repetition suggests that multiple cross-over and gene conversion events have occurred within a 1.0-kb region. Molecular clones of this sequence represent essentially the longest allelic form of this region seen in Southern transfer analysis. This repetitive DNA is similar to a sequence 5′ to the human myoglobin gene.