Myeloma mutant with a novel 3' flanking region: loss of normal sequence and insertion of repetitive elements leads to decreased transcription but normal processing of the alpha heavy-chain gene products.

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We isolated and characterized LP1.2, a mouse myeloma mutant with a deletion of at least 4 kilobases (kb) immediately 3' of the alpha gene and introduction of at least 5 kb of novel (nonimmunoglobulin) sequence in its place. A 6.2-kb genomic EcoRI fragment from the mutated allele was cloned, and a subfragment was sequenced. The deletion begins 11 base pairs (bp) beyond the normal site of cleavage and polyadenylation for the secreted form of alpha mRNA. A short direct repeat, eight copies of the 17-mer GCCT ATAGAAGTAAGGA, is located at the junction of the alpha and novel sequences. The first 4 bp of the 17-mer are identical to the last 4 bp of the alpha sequence. Novel sequences downstream of the direct repeats in LP1.2 include a low-copy-number sequence flanked by two distinct, highly repetitive elements. The low-copy-number portion of the novel sequence appears on a single 30-kb EcoRI fragment in several myelomas and in liver DNA; one copy of this fragment has rearranged in cell line W3129, and this allele has rearranged a second time in LP1.2. LP1.2 contains low levels of apparently normal alpha protein and mRNA. The S1 nuclease protection of nuclear and cytoplasmic RNAs shows that cleavage and polyadenylation are efficient and accurate and that they occur without the accumulation of aberrant transcripts. Alpha transcription in isolated nuclei is decreased sevenfold in LP1.2 relative to its parent, which accounts for the low steady-state levels of cytoplasmic alpha mRNA and protein in LP1.2. Decreased alpha transcription could result either from the deletion of a positive regulator in the 3' flanking region or from the introduction of novel sequences which exert a negative effect.