The sequence of the human immunoglobulin μ-δ intron reveals possible vestigial switch segments

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Abstract
We present the full sequence of an insert of a lambda phage clone which contains a segment of human DNA stretching from the secreted mu(mu s) constant region gene through to the beginning of the constant region gene and including the membrane mu(mu m) segments. The segment of 8.6kb extending from mu s to the first constant domain of delta(C delta 1) has been completely sequenced and reveals little conservation in comparison to the corresponding mouse sequence. The outstanding feature of the mu s-mu m intron is the occurrence of a potential Z-DNA forming region situated at 285bp downstream of the mu s poly A addition signal. A similar DNA stretch exists in mouse and may represent a site for transcriptional control of mu gene expression. The mu m-C delta 1 intron is much longer (6Kb) than the corresponding mouse intron and includes a series of different repeats, which start at 430bp downstream of the mu m poly A addition site and continue for 3.5Kb, ending about 1.5Kb from the beginning of C delta 1. This series of repeats may be a vestigial switch sequence used in the production of the secreting cells which are the progenitors of the rare human IgD myelomas.